If your lookup table is like this:

Cust ID       Name
000121       Joe Smith
000345       Fred Bloggs

etc, then VLOOKUP will not work, as the lookup value needs to be in
the left hand column of the table. You could duplicate the Customer ID
column like this:

Cust ID       Name             Cust ID
000121       Joe Smith        000121
000345       Fred Bloggs     000345

then use VLOOKUP on the name, or you can use an INDEX/MATCH
combination, which will allow you to get the ID from the left of your
name, as in the first table.

Hope this helps.

Pete

With Vlookup(), the lookup value must be to the left of the lookup array
containing the values your looking to return.
So, in your case, the name should be in Column A, and the customer number
must be in any column to the right.
If your datalist has the number in the left most column, then you'll have to
use the Index - Match combination.

You index the column that you want returned (number col.), and use Match to
find the pertinent row in that column, which references the column
containing the name.

For example:

=Index(C1:C100,Match(K1,D1:D100,0))