I would guess that, because of the drawbacks (no +$2, so no self-synergy; no attack) you will get away with $4.
Also other combos are not so strong, so f you play Vassal+disaperaing Village+Terminal money, that also does not combo, because you have a 4 card hand after Vassal, minus 2 cards Village/terminal, minus Vassal, minus card to set aside, so there is nothing to discard, so Vassal does not trigger anymore.
Of course it works with "real" Villages, but you drop half of the Villages out of this combo, so maybe just drop that clause, I don't think it's too strong if you get +3Cards when you discard nothing, especially as you need some cards to combo with, just by itself it's not a very good card. Best card out of 5 + 3 randoms is not that much stronger (if at all) than 5 random cards.
Edit: Best use (as it's now) is probably in Cantrip engines, you can clean your hand when it has run full with stopper cards and start with a fresh 4 card hand...
Edit2: I like the concept...
keep in mind that you dont have to set a card aside so if you maybe only have vassal and estate in hand you can just discard the estate and draw new cards.
but i have a idea to get around it and make the card a bit stronger
Vassal -4$ - action
+1 Action
you may put 1 card from your hand on top of your draw pile.
Discard your handcards. If you discarded at least one card: +4 Cards
is this ok at 4$ or does it need a penalty maybe?
I thought of gaining a estate when you gain this. this could be good in the endgame if you sadly only hit 4 and does hurt a little in the early game and hinders you to get them in masses without trashing.