The longest article I can find on the Vegans (outside of their treatment in GT:ISW) is a writeup in the MT
Imperial Encyclopedia. It's about half to two-thirds of a page in length, and goes into some physical, historical, and cultural detail, but doesn't provide any character generation information at all. It's really just an expansion of the original Vegan articles from the original
Solomani Rim and
Library Data supplements, though, so if you have them but not the MT:IE article, there's not that much that you're missing out on.
I never recall ever seeing an article on the Vegans in any issue of the old
JTAS either, and this
online article index seems to back me up on that.
It does seem odd that such an established and eminently playable race as the Vegans haven't gotten much more than a travelogue-brochure level of character treatment. My knowledge of
Challenge magazine articles is pretty weak, however, so they could have gotten something added in there and I wouldn't know about it. It also occurs to me that there might be a slim chance of DGP slipping something past us on the Vegans in their old
Solomani & Aslan book. Slim, but possible.
I don't have that book, however, and since it's nearly impossible to get at a price that would make anybody but a completist fanboy gag, I'm not likely to find out what's in it any time soon.