In many large companies there are monitary value limits on "gifts" from vendors and others. I know in mine it is $25.00. Now the occasional business dinner is allowed but it had better be business related such as vendors in town for meetings or you in the vendor's town, etc. Not just steak dinners for the sake of "being nice." If it is an extended event I make sure all expenses are equalized so if they buy dinner on Monday, I buy on Tuesday.
It is possible and probable that the vendor in question meant no harm in the offer and had no ulterior motive behind it. However it sounds like it was an offer well above any usual monitary limit so from a purely business ethics standpoint I personally would have gracefully declined. Not happily, but gracefully.
Now that said, I have gone out on excursions with vendors such as a weekend golf game, but I have always paid my own way and kept the receipts. LabRat was correct in the amount of influence one person in the right position can have so sometimes you do have to take extra pains to keep it above board.
I never let schooling interfere with my education. Mark Twain