That's what I figured, but in reading up on both of those techniques, I kept stumbling. It seems like the VLookup formula required you to have a specific criteria in mind (a particular number of text string.)

How would I design my VLookup, with row 1 on both my main sheet and the source sheet containing headers.

Main Sheet:
Column A: Drop-down customer list
Column B: Where I want to return customer number

Source Sheet:
Column A: Master customer list populating the drop-down list.
Column B: Customer Number