I don't understand your worry. Variables are used to not force you to apply changes in multiple places.
If you ever want to change the set analysis selections, you only have to modify the SET statement for variable v_Ignore_Some_selections.
If you ever want to change the expression result (by adding another Sum() or so), you only have to change the SET statement for variable Var.
Or am I missing something here?
I'm not sure what do you trying to do but maybe something like this is helpful:
vExp: sum($(v_Ignore_Some_selections) $1)
Var = rangesum($(vExp(A)),$(vExp(B)))