Ok, just checking. So the first IF function is supposed to check $B$17; if it's blank, then the IF statement should return a blank, otherwise it should do a VLOOKUP in some table for the value in $B$17. The IF, then, says

=IF($B$17="","",lookup)

I don't know what the exact VLOOKUP is, but let's pretend it looks like this:

=VLOOKUP($B$17,OtherWorksheet!$A:$C,3,0)

(That searches in column A of a worksheet named OtherWorksheet, looking for a value that matches $B$17 in this worksheet; if it finds a match in A27 (say), then it returns the value in C27.)

To combine them is simple; you just put the VLOOKUP call inside the IF call, where I wrote "lookup" before, like this:

=IF($B$17="","",VLOOKUP($B$17,OtherWorksheet!$A:$C ,3,0))

Note that you don't duplicate the '='; that's not actually part of the function call, it only introduces to Excel that you're about to do a calculation of some kind. But everything else is copied exactly, including all the parentheses.

Make sense?