vlookup formula based on formula issues

Hi

I've got several vlookups working to display information from different sources. My results show leaving dates (where found) in 2 separate columns (both based on vlookup formula)
E.g.
=IFERROR(VLOOKUP(A331,'May 2011 off Roll'!$A$2:$F$341,5,FALSE),"")
=IFERROR(VLOOKUP(A330,'Oct 2011 off Roll'!$A$2:$F$2843,6,FALSE),"")

I now want to display the leaving date in 1 column (rather than the 2 separate columns from my vlookups) and have tried several different formula but can't quite get it right. I've tried the following:
=IF(ISBLANK(U330),X330,V330).
=IF(U330>1,V330,IF(W330>1,X330,"")) - I've also brought through another value using a vlookup that I thought may help - no luck though.
I guess it's because although my result doesn't show anything and 'looks blank', isn't actually blank as the formula would normally bring back #N/A.

I'm not sure if I'm making any sense with this???
thanks