Can someone tell me the rules of lists used in vlookup?

I have two sheets with the same, unsorted list (sorted differently in both
sheets). I would like to use these list for the vlookup-formula, and when the
reference I use is on the top of the list (in the formula), it doens't return
the correct value (0).

Is there any solution to this without me having to add an empty row for each
new sheet or to do the actual sorting before I can use this formula?