Sort with ISNA and vlookup

[Tom]

I am having a problem sorting a spreadsheet that uses the following formula:

=IF(ISNA(VLOOKUP(ET$6&ThisSheet!$D8,'NextSheet'!$A$2:$D$3000,4,
FALSE)),0,VLOOKUP(ET$6&ThisSheet!$D8,'NextSheet'!$A$2:$D$3000,4, FALSE))

When I attempt to sort, the sheet, it puts the items in the correct order,
but the sort criteria no longer corresponds to the reference column.

For example, if the original data was:

C 2
A 3
B 1

If I sort by the first column, it will be sorted correctly (A, B, C), but
the values from the second column no longer correspond, e.g.,

A 2
B 1
C 3

Where am I going wrong?

Thanks.