"Yoga Vasistha" has nothing to do with the physical aspects of Yoga i.e. Asanas (though some passages do talk about them) but deals purely with the internal/intellectual aspects of the mind. It is a very long book with lots of Allegories/Similes/Stories/Repetition and so you have to read it slowly and with patience.

I recommend reading "The Concise Yoga Vasistha" (an abridged version) before reading the unabridged "Vasistha's Yoga" both translated by Swami Venkatesananda.