The problem is that the three minor virtues don't just cancel it. If that was all, it would be balanced.
Instead, one minor virtue (performance magic, music) is enough to cancel it. The three minor virtues combined+the flaw=the three minor virtues.
If you take a vis source, and waster of vis, your mage needs to be careful of vis expenditure, and uses up their vis source through their wastage. You don't use up your ability to be silent and still when casting.