I basically want to do a vlookup like you can in excel. 

We have a custom module called Sales Rep.  In this module the Sales Rep is connected with Vendors. 

In Contacts I have a lookup for the Sale Rep.  I want a field that will show Vendor Name of the Sales Rep field that is selected in the Contact.

Can I do this with a formula or do I need to create a function?  In functions would this be an Automation or a Related List?  I've never worked with either before so I don't know where to start.