If the population is under 100, and half of them are vendors, I find that unacceptable. I'd accept ~30 max' for vendors in that case.
Around 150, 200, under 60 is preferable.
Once we start hitting 400, it's OK for half of those to be vendors, but not more.
If I log in and find that there's no people, and that there's just a ton of vendors (I try to avoid trying these servers altogether - if they don't have a "Who's Online?" I usually won't even bother joining) I get pissed :x
The rest of the population? Up to 10% ought to be dual clients, max. If there's 50 people with 25 of those being an extra client, well, that's only 25 "real" players and 50 vendors. Lopsided.