I can't honestly vouch for a vegan/hardcore hindu diet, but I am pretty experienced with the typical 'vegetarian' diet, and I know for a fact that - while not necessarily optimum - real gains can be made in muscle mass and real strength.
Your typical vegetarian (and let's dispense with the debate over fish and seafood in general) is a big, big fan of soy.
Though it is much maligned, I can tell you that using soy in place of dairy will still yield solid gains.
I do not use it any more (PM me for more info) but soy protein isolate powder still worked for me just as much as whey, pea and hemp protein powders have worked for me.
For the record, though, it was whey protein powder that screwed up my digestive function and ultimately put me in hospital. I've learned a whole lot since then by experimenting with all of the protein powders I've mentioned above.
W.A. AMATEUR STRONGMAN
Axle clean-press: 100 kgs (220 lbs)
Bench press: 135 kgs (298 lbs) - 1st PL meet 16th October 2011
Deadlift w/Barbell: 180 kgs (397 lbs)
Deadlift w/Hexbar: 225 kgs (496 lbs)
Farmers walk: 240 kgs (530 lbs), 50 feet
Front squat: 100 kgs (220 lbs)
Log clean-press: 100 kgs (220 lbs)
Strict OHP: 85 kgs (187 lbs) 3 reps
Tyre flip: 260 kgs (573 lbs), 100 feet