B12 is probably the only thing you can't get from a vegan diet,
but you don't need much--it's likely you'll get enough just from
the things you don't check out carefully (the occasional baked
good with egg or gelatin or milk). Most vegans take supplements
but probably don't need to. Protein is not a problem, and the
old Lappe nonsense about carefully mixing protein sources to get
"balanced" amino acid levels has been debunked, so there's
nothing inherently wrong with a vegan diet.
But, since there is actually no valid evidence that a vegan diet
is better than a more well-rounded low-fat diet, I think it would
be negligent to recommend it to anyone except for religious
reasons. I am a lacto-veg myself, but I don't go out of my way
to ask every restaurant if they use a little lard in their beans
or gelatin in their desserts. It just isn't worth the effort
for unproven gains.