“Emptiness is not a specialty of Vajrayana. Yidam meditation is part of Vajrayana, but is not the special quality of the Vajrayana. The special quality [that enhances the practice] really is the bliss.” — Gelek Rimpoche 
There is no Vajrayana Buddhism without the concept of bliss. In Buddhism generally, the path to Enlightenment begins with wisdom, or insight — starting with understanding the four noble truths. This level of wisdom is basic, necessary, and foundational. To paraphrase Spock from Star Trek, that is “the beginning of wisdom, not the end.”
In Mahayana, compassion combines with wisdom to help prevent obstacles — incorrect views such as nihilism — by focusing on the wisdom comprehending emptiness and compassionate action (means).
In Vajrayana, the faster path, bliss, and emptiness become the cipher — and method — by which we try unlock realizations or even Enlightenment.
The Dalai Lama emphasized: “In all the manuals on tantric practice, the wisdom of bliss and emptiness is referred to repeatedly… All phenomena that appear are the manifestation of wisdom of bliss and emptiness.” 
Or, to quote an earlier Dalai Lama (the 7th):
This play of this divine mind,
The union of bliss, the supreme father, and emptiness,
Is unlimited and thus beyond concept.
This verse from Lama Chopa (translated from Tibetan) emphasizes the importance:
Inspire me to perfect transcendent wisdom,
Through practicing space yoga in equipoise on the ultimate,
Joining the bliss of supple ecstasy
With the insight that discriminates what is. 
In our daily practice, the Dalai Lama recommends: “Cultivate a state of mind focused on bliss and emptiness as forcefully as possible…This emptiness is the ultimate truth and also the ultimate virtue. And the wisdom of great bliss is the clear light wisdom: With a feeling of joy, imagine that offerings having such a nature pervade entire space.”
The most learned Vajrayana master Pabongka explains the importance:
“Within that bliss, the subtle primordial mind observes the object, emptiness. This is the most difficult, very subtle point of Vajrayana, the union of bliss and void.”
Bliss and Void as stage and base
The most venerable late Gelek Rimpoche explained it with a stage-play metaphor:
“Let’s say I am the Vajrayana, I am sitting on the stage. If there is no stage, I can’t sit on the stage, right? The stage of Vajrayana is bliss and void. If there is no void, you have no stage. Bliss and void are the Vajrayana stage, the Vajrayana base. All the performances that are done in Vajrayana are done on the stage of bliss and void.” 
He adds, later, “there is no difference between the Mahayana, Theravada and Vajrayana void [Emptiness]. Vajrayana may have a few techniques which may help you to understand it better or quicker, but it is the same void, not a different void. For this reason Vajrayana will emphasize particularly the development of bliss rather than the development of voidness [Emptiness].
The feeling of bliss overlays everything in Vajrayana practice. Even when we describe the Purelands — which many people think of as a state of mind — we often use the word “bliss” to describe it. It’s peaceful and blissful. Not just blissful, but the ultimate form of bliss — an ecstatic, perfect bliss. Not a temporary bliss, like that of an orgasm, but permanent, sustained bliss that only comes from realizations of the true nature of reality.
The key difference between Vajrayana and other paths
To use a different, although imperfect metaphor, if we are building a house:
- Beginnings of wisdom: we start with a really solid foundation, the best possible — the four noble truths and eightfold path. Our practice would be mostly (and importantly) Samatha (calm abiding) and Vipassana meditation (penetrative insight) — the foundation or insight path.
- Compassionate wisdom: in our metaphor, we architect a spectacular, livable, warm home for that solid foundation: bringing compassionate wisdom in action into our lives. Our practice would include Bodhichitta meditation, loving kindness meditations (metta) and other Mahayana practices — the Bodhisattva path.
- Bliss and Emptiness as path: we are ready to raise a roof that can withstand even the most violent of tempests and storms (our doubts and other obstacles). Our practice will now include “dress rehearsals” and role playing to help us progress (and be ready for those obstacles). These include guided meditations of bliss and emptiness (sadhana), deity and mandala, to help us shake our attachment to the persistent illusion of relative reality. With an understanding of ultimate reality, we achieve bliss. Sustained bliss and wisdom comprehending emptiness literally is a state of enlightened realization.
The final key in the treasure hunt: bliss
If we were treasure hunters, Indiana Jones style, “Beginnings of Wisdom” (Vipassana and Samatha) would be like following a trail of wisdom clues — blocked by many obstacles and antagonists along the way.
But, like Indiana Jones, we’d continue against all odds until, ultimately, many adventures later, we’d find our path transformed by compassion and the natural comprehension that comes with union of compassion and wisdom (Mahayana).
Finally, we’re at the door, and inside is the treasure, but we realize the door can only be unlocked with a password — this requires insight. The password can only be solved by bliss comprehending emptiness (Vajrayana).
Dharma texts symbolic of each stage
If we were to pick single Dharma texts to mark these stages, the keys would be
- Beginnings of wisdom: the first sermon: four noble truths, then the eightfold path
- The middle, balance of compassion and wisdom understanding emptiness: Prajna Paramita Heart Sutra: “Form is emptiness; emptiness is form.”
- Bliss and Emptiness as path: Tantra and Terma: tantric revelations; overcoming the persistent illusion of relative reality and comprehending ultimate reality.
Why non-dual bliss and emptiness? Insight requires both
Why bliss and emptiness? Lati Rinpoche, a most venerable teacher, explains: “sentient being in the sense of those beings capable of feeling and thinking.” Wisdom (thinking) realizing emptiness, combined with bliss (feeling) is a wholesome approach to the path. To use a “Star Trek” metaphor, logical thinking alone (Vulcan) can lead to conclusions without context.
In one early Star Trek episode, The Immunity Syndrome, a giant one-celled creature swallows entire planets and destroys a ship fully crewed by Vulcans. Spock explains to Doctor McCoy why they failed: “Call it a deep understanding… but I know, not a person, not even a computer on board the Intrepid understood what was killing them, or would have understood it had they known.” Why? Because logic alone is not insightful. Insights come from “feeling.” This is why, in Vajrayana, union of Bliss and Emptiness, or union of Compassion and Wisdom, are always compulsory. One should not be practiced in absence of the other. Famously, in one of the Star Trek Movies (The Undiscovered Country), Spock says, “Logic is the beginning of wisdom… not the end.”
Why is bliss important?
The main reason we need bliss is to help us cultivate insights. If all we ever do is meditate on other people’s revelations and insights — studying and contemplating sutra for example — we may not need bliss. Simple contemplation and wisdom suffice. However, to go the next step, to develop our own insight, bliss is critical — because understanding Emptiness too often leads to nihilism, an incorrect understanding of the concept.
Stated bluntly, meditation on emptiness usually leads to nihilism, which is not only an obstacle, it’s a major error of understanding. Simplified guided meditation talk about visualizing space as an early way of conceptualizing Emptiness. This is a poor choice, generally, as it tends to lead the mind to a nihilistic conclusion — especially in absence of insight.
Bliss does more than help avoid nihilism
Bliss helps us avoid nihilistic attitudes. It does much more than that, of course — particularly its function in facilitating deity pride, and its vital role in completion stage practices — but not too much of that can be discussed without empowerment by a lineage teacher.
Of course, it’s not that simple, which is why, on the advanced path of Vajrayana, a teacher is a must and empowerment is crucial. We are limited, as always, in how much we can reveal. Vajrayana deals with “both the coarse and subtle yogas of the generation stage and is meditating on the subtle drop at the lower end of the central channel or secret space, is able to bring all of the winds into the central channel” explains Lati Rinpoche. Even the description sounds cryptic. This is why, “how this is done” requires a teacher of lineage.
In general terms, we can explain that we seek a “state of meditative equipoise one is meditating on the wisdom of non-dual bliss and voidness or emptiness,” says Lati Rinpoche. “As one comes out of that meditative state in the post-meditational period one tries to see every appearance of whatever object one experiences as the nature of non-dual bliss and voidness. Also during the post-meditational period on this level one experiences this non-dual union of bliss and voidness in the form of deities.”
 Cittamani Tara Teachings: Gelek Rimpoche Jewel Heart Sangha (PDF)
 The Union of Bliss and Emptiness: Teachings on the Practice of Guru Yoga, by the Dalai Lama