I think there's some over-reacting going on.
If I see someone eating something that looks interesting and I feel I know them well enough, I'll ask what it is.
It would create a reasonable assumption that the person eating it might be a vegetarian. What's wrong with asking? It's not like I know the two other people ahead of my have asked the same question.
I might answer with "No....but if I was, this would be my main staple...I really love it."
No wonder no one wants to start up conversations anymore.
You also don't know that the two people ahead of you haven't asked the same question. If a question or remark seems obvious, whether it's "nice weather" or "you're very tall" or "are you a vegetarian?" you probably aren't the first person to ask it, maybe not the first person that day. At best, you might be the first one to ask it without following up with "did you play basketball in college?" or "why don't you eat meat?"
I don't mind having two people within five minutes ask me "is this where I get the train to the airport?" because that's impersonal and each, separately, has a practical use for the information. But it's not useful for a stranger to know whether I'm a vegetarian; it might be useful for them to know whether my lunch is, but "Is that vegetarian? I'm looking for more good vegetarian options" or "Is that vegetarian?" followed by "Can I get some around here, I'm looking for a lunch spot" makes it impersonal or about the asker: rather than demanding information about me, they're offering information about themselves.
There are lots of ways to start a conversation that don't start by asking for information about the other person's lifestyle or health.