Why? Yes, VS costs the same as retaliation, but it returns 1 rage and also has a 33% chance of resetting your rupture cooldown.
Your annihilate has a 66% chance of resetting rupture, which means a decent amount of the time you won't be able to piggyback your ruptures by using annihilate, and throwing a VS in there to add another chance to get your rupture back, I would imagine, would lead to a potential dps increase over choosing retaliation.
I could see using it though if your annihilate successfully reset your rupture cooldown, because then you could just perform an assault at the same time. You would get 1 more net rage by using retaliation+assault instead of VS, plus it should do more damage. Huh, I never thought to try that O.o