You have a couple of options for a workaround: one is to nest the
VLOOKUP into an IF formula, so the logic flow would be: IF(vlookup
value = "","",normal vlookup)

Another way to do it is to nest a MATCH function inside an INDEX
function, best shown by example: suppose values in A1:A4 are
penny
nickel
dime
quarter

....and suppose values in B1:B4 are
..01
..05
..1
..25

Copy the A1:A4 values into D1:D4 (to show that the values can appear in
a different cell, must be spelled exactly, etc). Then in cell E1,
enter this formula:
=INDEX($A$1:$B$4,MATCH(D1,$A$1:$A$4,0),2)

The English translation is: find an exact match of D1 in the range
A1:A4 (which becomes a row reference for the INDEX function); in the
range A1:B4 go to the row specified by the MATCH function and the
column number supplied (i.e., 2).