Vegan cats . . . shiver.
I suspect they'd be fine, assuming you gave them things like avocado and such. I'm not sure though that the bagged foods are actually vegan-safe . . . I'd have to read the back of them. But rats are pretty flexible eaters, honestly. I wouldn't put mine (technically my fiance's on a vegan diet) but I doubt it would hurt them . . . heck you could probably just give them what you (the vegan) were eating and they'd be ok.
On the other hand, I have to agree about enforcing your ethical beliefs on your pets. But then, I'm not a vegan or a vegitarian, so I probably can't discuss that issue from a legitimate standpoint.