Auto-populate corresponding info w/ Data Validation

I'm wondering if there is a way to use data validation to automatically populate an adjacent cell, given a particular selection of valid data
.
For example, the user selects account 1001 from a drop down that uses an outside file for data validation. The adjacent cell (next to the one now showing 1001) should auto-populate with the account name, provided that the account name is in the column next to the valid data in the outside file.

I suppose a vlookup would work here, but is there another ("cleaner") way to do it? I don't want to have to update formulas or worry about users messing with the formulas.

Why not just protect that adjacent cell so the user can't jack with the formula? One formula should be able to look up the account name in the other spreadsheet. I would tend to use INDEX & MATCH over a VLOOKUP, although I haven't seen your workbooks.
Add IF(ISERROR() to eliminate returning an error when the account number hasn't been selected yet or there is no account name associated with the number.


...if you don't want a formula in the adjacent cell you would have to use a Worksheet_SelectionChange procedure to find and load the account name into the adjacent cell each time the data in the "account number" cell changes.

your user must have macros enabled
and if by chance your workbook is shared, it will slow to a grinding halt!