The Gheranda Samhita describes vrishasana as placing the right heel under the perineum and the left heel over it, and turning onto the side. It is one of the very few postures in the classical corpus performed lying on one flank rather than seated, prone, supine or standing.
That makes it a good illustration of how the later manual widened the repertoire. The Hatha Yoga Pradipika's fifteen are seats, two arm balances and a supine rest. The Gheranda Samhita adds prone forms, kneeling forms, standing forms and this one lying on its side, and the additions are what take the list from fifteen to thirty two. The earlier text does not name vrishasana.
The heel placement is also worth noting, because it is drawn from the same vocabulary as the seated group. One heel under the perineum, the other above it, is the arrangement that distinguishes siddhasana, muktasana and guptasana from one another; here the same arrangement is carried over onto the side. The compiler is not inventing a new set of relationships between the limbs so much as taking established ones into new relations with the ground, which is a pattern visible across several of the postures unique to this text.
Side lying is close to unique in the classical corpus. The manuals record seats in quantity, prone forms in a small group, two arm balances, one supine rest and a few postures on the feet, and then this. Whether that reflects the range of practice or only the range a compiler thought worth writing down cannot be settled from the texts, and the atlas records the distribution without proposing an explanation for it.