Alan Wallace, in his essay External, Internal and Nondual Space, explores vacuum in those three spaces.
In external space, there are two forms of vacuum: relative vacuum (the approximation of vacuum we can create with our current technology) and absolute vacuum (space empty of anything it can be empty of, within the current habits/setup of the universe).
In the same way, there are two forms of vacuums of consciousness.
The relative, bhavanga, or individual ground of becoming, is that which the dynamic content of the individual mind emerges from. He notes that it has the lowest possible kinestetic energy state (no movement), but the highest potential energy (pregnant with possibilities). It is associated with experiences of bliss, luminosity and nonconceptuality. This can be experienced through meditative practices that helps the content of the mind quiet down.
The absolute vacuum is the dharmadatu, out of which emerges all phenomena – intersubjective and intrasubjective. The experience of this is called primordial consciousness, or jnana. It is fully non/transdual, beyond and embracing subject and object, experiencing and experienced. It is the One Taste. This can be experienced through contemplative insight or inquiry.
It is interesting that my experiences over the last couple of weeks of a fuller transduality, of there only being whatever is happening in the present with no distinction of experiencing and experienced, came through inquiry – Douglas Harding’s safety helmet experiment. And the relative form is something I am familiar with through sitting practice.
Each tradition has its strengths, and the strength of Buddhism – and in particular Tibetan Buddhism – seems to be its comprehensiveness and accuracy.