I really don't think I've ever heard any of this 'preaching' and evangelism from vegetarians, and I have known, and do know, quite a lot of them. Maybe it's different elsewhere, but it just doesn't seem to be common around here. I think most vegetarians I've known are such for what you would call ethical reasons, in that they just don't want animals to be killed for their food. I don't see how anyone can argue against this (aside from trotting out what always feels to me like a naturalistic fallacy), and it rather surprises me that they aren't more evangelistic if anything, since if you actually feel that it's 'wrong' to kill animals for food, I would think there would be a moral imperative to spread that to others, but I just don't hear it, presumably because they're adult enough to realize that if others shared their beliefs, they would already be vegetarian.
I wouldn't say I've heard evangelising, as such, in the other direction, but I know that vegetarians often have to put up with constant teasing and cajoling to eat meat, as well as frequent questioning as to why they eat as they do. I suspect most vegetarians have also had the experience of someone coming up with ludicrous hypotheticals along the lines of "what if you were stuck on a desert island with just a herd of pigs for food? Ah, so eating meat isn't necessarily wrong! etc..." I have also known meat eaters who will serve meat to vegetarians or dairy to vegans and tell them that it is veggy, presumably on the basis that what they don't know can't hurt them.
I'm 30 and have been a vegetarian since I was 11. I admit that in my first couple years as a veggie I did have "meat is murder" stickers on my trapper keeper and, if the subject was brought up, would passionately argue that meat eating is immoral. But I grew out of that within a couple years, and frankly I was a kid, so I don’t think my brief "evangelical vegetarian" period should really count.
Now I feel that humans are omnivores. I do not think there is anything inherently immoral about eating meat, I buy (humanely raised) and cook it for my husband and dinner guests. I just personally care about animals more than I like the taste of meat, so I chose to not eat meat because I am able to get all my nutritional requirements without it. It also helps that I was never a big meat eater to begin with, so it's not a challenge to abstain from it.
I have known many vegetarians and the vast majority of them have not been "evangelical" vegetarians who try and convince others. Also, of the few I do know who were "evangelicals," most of
them were only that way in their younger years and grew out of that phase fairly quickly. However, I'd say about 50% of the time when someone finds out I am a vegetarian, the person I tell criticizes me, teases me, tries to convince me I'm being stupid, etc. Yeah it's true that PETA is a big publicity hound, but I see way more ads from, for instance, The American Beef Council than for any vegetarian groups. In my experience it's overwhelmingly meat eaters trying to push their lifestyle on vegetarians, not the other way around. Yet whenever vegetarianism gets brought up in this forum, there is invariably meat eaters complaining about vegetarians "pushing their lifestyle upon them."
Not that I'm complaining, because it honestly doesn't bother me one bit when people get on my case about it. I find it amusing more than anything else. The best comment I ever got was in the North End (Boston's Little Italy) awhile ago buying cheese at a cheese & butcher shop. The old man behind me asked me, after ordering just cheese, "Hey, aren't you going to buy some of that delicious sausage they've got?" To which I replied, "Oh, I'm a vegetarian." His response was an exaggerated eye roll followed by "Pffft, I don't need to hear about your personal problems."