Look sub.... we could smash through definitions of practically ANY label on earth, given enough deconstruction on how people choose to use those labels. Given the fact that many people would probably eat human flesh (from fallen comrades) if it were the only means of survival (this has happened), would we revise our earlier assessments of these people, pre-plane crash, and say they were cannibals, because it was not a sure thing that, given the right circumstances, they would absolutely, till death, refuse to eat human flesh?
We're looking at reasonable definitions here. Obviously, someone who claims he's quit smoking.... yesterday... can't be considered a non-smoker or, at the very least, someone-who's-quit-smoking. That would be absurd. By reasonable standards, one would say he/she'd have to spend some time in a smokeless state, say a couple of months or years, and most people would probably be comfortable calling him a non-smoker* (who used to smoke before but doesn't now).
So, based on what you pointed out, I would say that if people eat meat under no compulsion to eat it (like a gun at one's head, or there being no other source of food, so death-by-starvation is a possibility) then they're not vegetarian. If I'm Muslim, I can still decide to be a vegetarian. There's nothing in the Koraan which says that Muslims have to eat meat. So there'd be a real and meaningful distinction between a regular Muslim who restrict his/her meat-eating to times when he/she has halaal food available and a Muslim who just never eats meat because he/she's a vegetarian.
As for the schizoid at the dinner party who claims to be a vegetarian even though he/she may have eaten meat a week before and has no strong commitment to avoiding meat the next day, it's like a smoker who claims he's quit smoking after having one day, or even a week, clean.
You've got "IYO" (or, since I'm speaking, IMO) all wrong.
IMO, there are two major criteria for defining someone as being something... generally... like a writer or an artist or a vegetarian or a non-smoker or a non-violent activist.... etc...
1) Proven track-record or history of living up to the defining characteristics of the label.
2) Commitment to continue embodying said defining characteristics.
[this does not include certain exceptional labels, like ethnic ones, where there is no choice involved in either living the life indicated by the label or of withdrawing voluntarily from that life... so ethnic Jews can't stop being ethnic Jews, though a vegetarian-from-birth can cease to be vegetarian by choice... the element of choice is crucial in the forms of labels that I'm speaking of...]
Having said that... onwards...
"Cos you can't define it by the past, since a person who has only taken the decision to be a veggie today is still a veggie until the moment they eat meat, IYO..."
No... there has to be a proven track-record of vegetarianism, as I've shown with the cigarette-smoking example. Additionally, you wouldn't ordinarily call someone who's blacked out once in his life an alcoholic (provided he hasn't abused alcohol consistently before in other ways). He may have done something alcoholics do all the time, but that doesn't make him one. Repeated patterns of self-destructive behavior over a significant period of time, like shirking work due to hangovers or repeated black-outs, would slowly build a case for his being an alcoholic, and that's when people would call an intervention. It would be ludicrous for family and friends to call an intervention solely based on one black-out (unless they're super tee-totalers).
"and it can't be defined by the present because if you're happy to say you might eat meat tomorrow then IYO, you're not a veggie."
It depends. If someone who's been a bona-fide vegetarian, meaning he/she's had a significant history of an exclusive non-meat diet for a long period of time (say, a few years), and he/she's happy to eat meat the next day, he or she's still a vegetarian until he/she breaks the fast. But then we get into situations where a long-time vegetarian tries out meat and then says, nah, not for me. Resuming the earlier lifestyle reflects a continued commitment to a lifestyle which is bolstered by a long history living said lifestyle. So that person would still be a vegetarian.
"So you're defining it by the future. Which is unknown. I just think that's a bit weird."
As I've clearly demonstrated, I'm not defining it by unknown futurity in any way whatsoever.
To break it down somewhat more formally.
There is an activity. There is an actor. Then there is a label for someone who has taken the performance of that activity into a continuous, even (partly) self-defining state, let us say Actor, or "actor" with a capital-A.
A regular person who performs activity-1 can, no, must definitionally at that moment be considered actor-1. But that doesn't make him/'her an Actor-1.
So, if I act in my school play, I'm acting in it, I'm an actor-in-my-school play.... but that doesn't suddenly make me an Actor, like Brad Pitt, someone's whose lifestyle is partly defined by doing that activity. Brad Pitt is an actor. Someone who earns his bread, or even does acting part-time but consistently over many years, may be said to be an actor. Little Timmy who plays Iago in his middle school play? He's not an actor.
Same thing goes for a vegetarian. Practically every human being on this planet consumes plant matter. But that merely makes them omnivores. "Vegetarianism" is a phrase that was coined in the 1800's to describe those people who decide not to eat meat and commit to that lifestyle in a meaningful way.
We say vegetarian food because the meaning of the word "vegetarian" has expanded to mean all non-meat foods, which are generally vegetables, fruits, legumes, and even fungi, which aren't even plants. So someone who generally eats vegetarian food but has no problem, uncoerced by forces majeures or sigma-situations (meaning, situations falling well outside of the norm), in eating a beefsteak and regularly eats meat (say, on a weekly basis) can't really be said to be a vegetarian.
Now what of a vegetarian who's vegetarian all year round, but eats one big steak dinner on Christmas? So he's 40 years old, and has eaten, say, 35 steaks his entirely life, and will continue to do so... is he a vegetarian? Well, kind of yes, kind of no.
This is the problem with the semantic game you and I are playing, Sub... we can forever slice the spectrum into finer and finer lines, and someone will always find another line which slices my line in half, because unbeknownst to me, I didn't manage to actually draw a line, which has no width, but rather drew a very thin bar-line, which encompassed several ever-so-slightly-distinguished model cases which could be deemed different or problematic.
I've frequently used the terms "reasonable", "repeated", "significant".... these are very big words with lots of space for quibbling. However, when we stop trying to be analytical, because achieving an intensional and extensional definition of "vegetarian" or any of these other terms would be highly difficult, and simply see how people use labels in general, we get the idea that there is some consensus on who deserves certain labels and who doesn't.
There is room for some sharp distinctions to be made in these zones between definitions, however. In my opinion, someone who regularly eats fish isn't a vegetarian. Etc. etc. etc.