How to deal with narcissistic manipulation from anti vegans?

What you describe as a lose-lose scenario is what's commonly described as a confirmation bias for a given belief (e.g. vegetarians are brainwashed). You can't work against that with conventional rhetoric, even if you win on the logical grounds, there is a big difference between proving something and convincing someone.

As for convincing someone, being from the other side of the fence that your interlocutor is can be a curse or a blessing. Many previously homophobic people had to put some water in their wine when a close relative outed themselves as homosexual, for example. But if you happen to be someone they dislike, they will use anything they dislike about you as a way to feed their confirmation bias and trying to prove them wrong is a good way to dig that hole.

There are two ways to work with a claim that has purely irrational grounds; either you point out the lack of rationale by inquiring about what the facts to support the claim are, which can be useful to convince an audience, or you directly skip the factual support thing and directly ask why, or since when, does one have negative feelings towards the community.

This lets your interlocutor explain the root of the belief around which you can provide your own, measured interpretation: "Well this was a bad apple, no vegetarian that I know would do such a thing, and don't you think similar things happen in other communities as well?". By displacing the conversation to be about them instead of you, you bypass the wall while still inviting them to change views.

1. You could question them. "What am I brain washed about?" Probe into the specifics. The devil's in the details ;)
2. You could draw comparisons to other movements, either historically or contemporary.

For the 'smearing tactics', continue to ask what they are talking about:

• For instance, "What vegan parents let their children die?" (The one case I heard about, it was a sensationalistic news article about parents who only fed their infant applesauce, water and something else. Idiot parents cut both ways.)

• "What were you judged about?" (There are many different outcomes for how they initially answer the question.) There are extremists in every movement. A few bad apples doesn't mean we should generalize to blanket statements.

• "What nutrients does a vegan diet lack?" (Look into the top nutrients people fret about.) Then go into how either fortified foods can be eaten or vegan supplements taken.