AFAIK, this is the only option you have with a list from a SQL Database. Not sure why you would hide the New Vendor Code control though. You could add some spaces between the vendor name and new vendor code and set the width of the drop-list control so that the users wouldn't see the new vendor code. That would make it appear that you are using an alias list. You can use the Find function to get the location of the [ charater so the number of spaces wouldn't matter.
Thank you Michael for your answer! Good to hear that I was on the right trail with this! I will set this up tomorrow at work.
The reason I want to hide the box with New Vendor Code is because I want to eliminate confusion and reduce the amount of fields that are visible for the users. This will be a non editable field anyway, and the PDM user doesn't really care about the code of the Vendor. All they want to do is pick the correct one, and what is going on behind the scenes is not important to them.