VLookup Problem

I am doing a VLookup of a column which mostly has numbers, but also contains some number/letter combos. For instance: 756, 856, 956, 9RK, 8FB. The VLookup will return valid data for the 756, 856, & 956, but when it gets to 9RK it will stop and return nothing. I already have an IFISERROR in place, have formatted the numbers to text, and sorted in ascending order. What do I have to do for the Vlookup to return correct value for the letter/number combos? Thanks for you help