I feel this is VERY inappropriate - it is widely done and most vendors are happy with it. But for my business, when this is offered to me, I tell my vendors to simply link me on their website or refer me to more clients, but I don't accept money!
It is completely up to you, of course on how you want to run your business, I just think its tacky since I only will refer the great vendors - NOT the paying. I feel If I referred the GREAT & AMAZING Vendors who made the night shine - that was money already! And I say, lets keep doing it! (BTW, Great topic!)
Good Luck to you!