Don't use conditional expression in filter that will not be perfect in sometimes
May be you can load the table b as Mapping load and use Apply map for this.
May be the script will looks like this
Load * from TableB.qvd(qvd);
Mapping Load TableB.Values, TableB.Descriptions Resident TableB;
Load *, ApplyMap('TableBMapping', TableA.VALUES, TableA.DESCRIPTIONS) as TableA.DescriptionsModified