You might need to run it a few times if you have a large number of
non-contiguous cells:
http://support.microsoft.com/default...b;en-us;832293

Sorry, running it a few times isn't the answer, use the advice provided at
the link:

WORKAROUND
To work around this behavior, you may want to create a looping structure in
your VBA macro that handles less than the maximum 8,192 cells.

I.e., pass the Vlookup() function to smaller ranges over several passes.

Regards,
Nate Oliver