VLOOKUP formula, cell reference as column index number

Hi I am trying to use a cell and enter a number that will be used as a reference instead of hard coding the number I formula below as Col_index_num.
.
=IF(B598="",Q598,VLOOKUP(B598,INDIRECT("'["&G598&".xlsx"&"]"&H598&"'!"&"$A:$AE"),8,0))

What syntax should I use?

Thanks.