As a side note, most people who I have seen get into training as vegetarians either give up training or vegetarianism given due time...but that's just my experience.
I know this was just a small side point, and I agree with the other points you were making, but I wanted to comment on this because it depends a lot on what type of vegetarian diet you use and why you do it.
If you eat eggs and dairy and just don't want to kill stuff (meat/fish), it's not too difficult to compensate nutritionally for that choice.
As far as I know, the only thing you are really missing out on is the Creatine in meat, and you get that even more easily from supplementation. Everything else is stuff you can get from even better sources (better and cheaper proteins from dairy and eggs and protein powders in general, better essential fat sources etc).
On the other hand, there is nothing about training that would make people just suddenly become morally inconsistent (choosing to apply their principles only when it's convenient), you are not suddenly going to kill puppy dogs and trip old ladies in the street just because you train, and similarly you wouldn't start killing/eating/hunting stuff if you didn't want to do it before.
If however you are a vegetarian -not- for ethical reasons and don't even eat eggs or dairy, it's harder to compensate for (still possible, just a lot more work), and I suppose it would be much more likely that you could change your mind (maybe if you thought being vegetarian was somehow healthier and later realized that while training it would be healthier and easier to eat meat, fish, dairy and eggs too).
So anyways, you might want to specify more details than just saying vegetarian, so people can give you more specific advice.
The word vegetarian can mean different things, if I just say I am vegetarian people might assume I like sophisticated vegetable dishes that involve mushrooms (which I hate), when I would be more all about chocolate, watermelon and frozen yogurt

That's another thing you might want to be careful about, generally it's easier to gain weight as vegetarian if you are careless, because the most delicious calorie-rich foods are technically vegetarian, so it's not in any way a guarantee to be healthier at all (think of potato chips, every possible candy, fried battered food, etc).