Cell reference in VBA code with Excel Functions

I don't use Excel that often, and VBA even less often, so I suspect I'm missing some simple concept. I have this line of code in a macro:

ActiveCell.Offset(0, 7).Formula = "=VLOOKUP($C21,Lists!$C$1:$E$99,3,FALSE)"

It works, except that it doesn't update the $C21 to $C22, $C23 etc. when called to create new rows. I can see that this is probably because the VLOOKUP arguments are quotes rather than dynamic cell references, but I'm not sure how to get around it. Any ideas?

Thanks.