i have the following formula which works fine:
IF(IF(ISNA(VLOOKUP($A4,'Copy of Source data'!$A$6:$ET$550,64,0)),"",(VLOOKUP($A4,'Copy of Source data'!$A$6:$ET$550,64,0)))=0,"",IF(ISNA(VLOOKUP($A4,'Copy of Source data'!$A$6:$ET$550,64,0)),"",(VLOOKUP($A4,'Copy of Source data'!$A$6:$ET$550,64,0))))

if the above formula returns a blank then i want to add another if statement to vlookup another parameter...i.e.

=if(IF(IF(ISNA(VLOOKUP($A4,'Copy of Source data'!$A$6:$ET$550,64,0)),"",(VLOOKUP($A4,'Copy of Source data'!$A$6:$ET$550,64,0)))=0,"",IF(ISNA(VLOOKUP($A4,'Copy of Source data'!$A$6:$ET$550,64,0)),"",(VLOOKUP($A4,'Copy of Source data'!$A$6:$ET$550,64,0))))="",vlookupetc..... other "")

Thinking about it, the result of a lookup can only be one of the following four options:

1) error because lookup value not found
2) zero because lookup value found but lookup column is empty or zero
3) lookup result
4) blank because lookup value found and result cell contains just an apostrophe or a blank string, ie one or more spaces. This could be as a result of a formula that resulted in "" or " " being copied and pasted as values.

To be blank it would be option 4. Is that option likely to occur?

Slight re-ordering of the checks was required, checking for blank before 0 because as mentioned blank would normally return 0.

=IF(ISERROR(VLOOKUP($A4,'Copy of Source Data'!$A$6:$ET$550,64,0)),"",
IF(ISBLANK(VLOOKUP($A4,'Copy of Source Data'!$A$6:$ET$550,64,0)),VLOOKUP($A4,'Copy of Source Data'!$A$6:$ET$550,44,0),
IF(VLOOKUP($A4,'Copy of Source Data'!$A$6:$ET$550,64,0)=0,"",VLOOKUP($A4,'Copy of Source Data'!$A$6:$ET$550,64,0))))

If lookup 64 is an error then blank else if lookup 64 is blank then lookup 44 else if lookup 64 is zero then blank else lookup 64

Alternatively, a sample workbook would be great.