This is correct, there is no "IN" clause.
You need to test your items within the brackets individually against vTestvalue. You can do this simplistically eg.
IF (
( vTestvalue @='ABC' %
vTestvalue @='XYZ' %
vTestvalue @='MNL' ...
You need to test your items within the brackets individually against vTestvalue. You can do this simplistically eg.
IF (
( vTestvalue @='ABC' %
vTestvalue @='XYZ' %
vTestvalue @='MNL' ...