First, you'll need to create two formulas, I'll call them {@Pri} and {@Isp}:
{@Pri}
If {DataTable3.vid} = 'Pri' then {DataTable3.Izno} else 0
{@Isp}
If {DataTable3.vid} = 'Isp' then {DataTable3.Izno} else 0
You'll then create a final formula that will do the rest of your calculation. (I'm assuming that you're doing this for the whole report, not some internal group.)
Sum({@Pri}) - sum({@Isp})
-Dell