At my company everyone gets 3 weeks, nonnegotiable. But they let you supplement it with unpaid leave, which works out to the approximately the same thing if you get a corresponding pay raise.
I guess other companies are probably frequently less accommodating.
I've managed, once, to get the full vacation time for seniority (i.e. At N years you get +1 week) by threatening to quit after being offered a 5 figure raise.
So I'd agree with you. I've never known anyone to get away with pushing past the upper limit on their seniority track for vacation.