Excel vlookup/IF formula advanced

I'll try to simplify and explain my situation.

I have an Excel table (1,000+ rows)from 2012 which is my basic starting point for 2015 calculations.

I need to compose nested function/ formula that 1. finds matching numbers/ codes of each employee - i would use vlookup 2. goes to cell XY and makes a test - if the cell contains formula, copy an exact formula from validation sheet. Otherwise, if cell contains number (just typed in) then copy that number. I would use IF formula.

However, can not construct the formula so it works and it would be of a tremndous help and save me loads of time. Besides, this is great opportunity for me to learn something new. Thanky you very much for any advice.

Best, fina