Formula to return a unique identifier to a row of data based on 2 columns?

[krups]

I am trying to find a formula to return a unique identifier to a row of data based on 2 columns. I found this one below but it uses 1 colunn only:

=IF(ISNA(MATCH(B7,B6:$B$6,0)),MAX(C6:$C$6)+1,VLOOKUP(B7,B6:$C$6,2,FALSE))


Last

Name
Smith Bob
Smith Mark
Smith Bob
Gil Tom
Thomas Bob
Furst Andrew
Smyth Markus
Reynold Tom


This formula returns a unique identifier to a row of data based on 1 column.
I am trying to find a very similar formula except using 2 columns of data, e.g.:

Invoice Number Vendor

I want add a unique identifier where the Invoice number is the same however the Vendor is different and if both are the same then use the same identifier.

Please help

Thank You