Using either vlookup or IF formula with one Data Validation drop down

Hi All, I'm trying to set up a spreadsheet where people can either type in an employee number, or select an employee's name from a list if they don't know the employee number.

I have a data source of all the employee numbers and the relevant employee names (example attached Book1.xls).

I would like to set it up somehow, so that if the employee number is typed in the first column, then the name will appear in the second column. I know I can use a vlookup for this - however - I also want the option, that if the employee number is not known, you can select this employee name from a drop down list in the second column, and then it will somehow populate the employee number in the first column.

This is where I keeping coming unstuck - I can do the vlookups on their own, or the data validation on it's own for a drop down list, but I just can't get it to all link together nicely.

Can anyone help? Thanks!

Thanks very much for the answer book - I'll have a play around with it. I really appreciate the help!