A vendor, as it is used in the strip, is a 3rd party business that wants to sell its services or product through Dilberts company. Often the term 'vendor' is used for companies that show up at a jobsite with fliers or snacks for various employees to promote their services. It could also be used to describe a 3rd party that does business on site like a plumber or gardener.
I guess that "vendor" can be "salesman", too. But since vendor is, as far as I know, a legal term as well I would guess that it is little broader in meaning than merely being "salesman". And somewhat narower at the same time, since "seller" would be approximately equivalent to "vendor", or am I all wrong here (as usual)?