You basically have two processes:
1. Users fill out the form where they select a vendor
2. A list of vendors must be maintained.
You could have a SharePoint list of vendors, for example, maintained independently of the form. The form would simply read the values from the ShP list. This would be the easiest solution to implement.
It IS possible to integrate the two processes i.e. have users add vendors while filling out the form. But that's not easy to explain or implement, and would likely require some code or at least some pretty advanced InfoPath design.