It can be done and here is a example.
=IF(A1="CS",VLOOKUP(B1,M:O,3,0),VLOOKUP(C1,P:R,3,0))
Vlookup 1 is if it is CS
Vlookup 2 is if it is NOT CS obviously i have made the vlookup just look elseware on the sheet you will need to put in your own vlook up. I hope this helps