The best argument you can make against being a vegan(not vegetarian) is that there is no natural source of B12(nutritional yeast isnt really easily found)
That said, there is absolutely NO reason at all a vegetarian would be at ANY disadvantage for either bodybuilding or powerlifting. Everyone who argues that there is uses irrelevent factoids to support their view. Until you can show me something that you get in meat that I can't get in a vegetarian diet, it's nonsense.
I'm a 19yo 5'8 180 lb vegetarian(been a vegetarian for 10 years) with visible abs unflexed, and a 1275 total.
Using anecdotal evidence of the variety most commonly spat out out by meat advocates, is about as fair as me comparing my lifts to untrained individuals and supporting vegetarians lifestyles as optimal for strength gain.