To me it sounds like the waitress was an idiot or was lazy. I would have asked to talk to the owner or manager, because I would need a very good reason to return to a restaurant that ran out of a kitchen staple like vinegar. If they never have any, I'd want to know why - because the most likely answer would be that the restaurant actually is making none of it's own food, and is actually just reheating ready-made stuff that they're getting in frozen (another good reason not to return to that restaurant).
As for salad dressings in a pinch. Salsa and sour cream also make a great salad dressing, but any restaurant that doesn't have vinegar, couldn't be counted on to have salsa and sour cream, either.
Yikes, I know I'm ranting, but I can't imagine a restaurant runnning out of vinegar.
Literally, though I do suspect the waitress was just new and doesn't understand special requests. When, I've made special requests, I've often had waitresses tell me "no, we can't do that." I (sweetly) ask them to get the owner or manager, or who ever's in change (sometimes it even ends up just being a more experienced waitress) and then ask that person - who always says "Of, course we can do that."
Then I usually hear the new waitress being told that they should ask someone if they're not sure whether or not a special request can be accomodated.
I understand the waitress not feeling she has the authority to approve a special request, but usually she'll then say, "I'll check with the ____ (manager, owner, cook....).
Sorry, I know the topic isn't restaurant service, that's just a peeve of mine.