Make sure you have your VendorID set as the same datatype in both the tables.
N.B: Even if it is stored as a number, make sure they are both long integers. The duplicates shouldn’t prevent you from creating a relationship:
duplicates restrict only when you try and create a relationship with referential integrity.
I had the same issue. In one table I had guestID as a primary key with the AutoNumber format, and had a 1 to many relationship with guestID in a different table. I must have put the format to Short Text by accident, so when I changed it to Number, it got rid of the problem.