Vasudans eat
headz simple as. That and sand and gin.
...and ramen noodles, pinto beans and muffins.
Erm... being a bit more serious - I think they drink water (something is mentioned about most of the above-ground water being undrinkable on Vasuda Prime, even by Vasudan standards, in the FS1 tech room; still it doesn't mean that they can't treat it). Maybe they live underground some of the time and eat whatever lives there, considering that the surface is a harsh place to live.
The two continents of their homeworld are said to be mostly desert, so that leaves some room for other biomes being present on the planet. Maybe even seas.
Given that their homeworld is so harsh, they probably adapted to enable them to eat just about anything.