So, im short on caps, but I have alot of things to sell. But every vendor I buy something from have just something between 800-3000 caps. Only the gun runners have more (around 8000 caps). But is it any vendors who have 20k caps or so?

No. Buy some useful stuff from them first, then sell your items.

Blake at crimson Caravan can sometimes get 12,000 after he restocks.

A good vendor in this game is one who is still working. Many during the course of a game break and stop refreshing.

Come to think of it I don't think Blake over at Crimson Caravan has ever refreshed on any character I've played since release. Whatever stock he has when I first talk to him is all I get for the rest of that character's life. Alexander over at the 188 as of the latest patch does this now for me....and I can't even use the "Sunday Bug" to force a refresh on these two.

I don't think any but the Merchant stuck at Novac is broken.