A vendor is typically someone with whom you have a long-term repeat relationship that you use to acquire certain things that you need to operate your business.
Typically this sort of long-term relationship that often can be kind of complicated and extends over a period of time is exactly the sort of thing that lends itself to having those sorts of details in writing. It creates a better relationship between you and your vendor because you each know what’s expected of you. A proper sort of vendor contract will spell out who is supposed to do what, by when, under what circumstances, over what period of time and how much it will all cost.
An attorney can help you make sure that your contract is clearly written and meets all of those requirements.