Hi, Kelli --
Welcome to the forum.
Will your form be creating projects as well as submitting vendors? Or will it just be for submitting the vendors to already existing projects? Can you give more info on what you mean by inputing the vendors in the most efficient manner? Do you mean in terms of actually filling out your form? Or the tables used for your form? Or something else entirely?
You can create a form based off two tables -- first select the parent table (yours is probably your project table) then adding a table in the data connection wizard to add the child (vendor) table.
Here's one link with some info that may be helpful. Here's another with some screenshots that shows setting up a relationship between your tables. I hope that helps, but if I misunderstood, please post back with more details...