Yeah... basically the problem is that all alt VP cards count something else. And most alt-VP cards only do better than Province at the extremes; Gardens needs 39 cards in deck to be a better buy than Province, Fairgrounds needs 15 uniques, Silk Road needs 15 VP cards, Duke needs 4 Duchies, etc. On the other hand, all (except Duke) are better than Duchy even at moderate values: 19 cards, 10 uniques, 8 VP cards, respectively.
So now you have a card that counts itself and must use itself as an extreme. But you don't have to buy both the card and the counted card, you only have to make one purchase. To make this work well, the first couple of buys can't really be worth much, if anything. You might be able to get away with decreasing every bonus, so something like this:
Card
$5*
When you gain this, gain a token. At the end of the game, remove two tokens from your mat. This card is worth 1 VP per token on your mat.
---
This card's cost is increased by $1 per token on your mat.
1 Card: 0 VP
2 Cards: 0 VP
3 Cards: 3 VP ($7 for 3 VP)
4 Cards: 8 VP ($8 for 5 VP - better than Duchy)
5 Cards: 15 VP ($9 for 7 VP - better than Province)
6 Cards: 24 VP ($10 for 9 VP)
7 Cards: 35 VP ($11 for 11 VP - better than Colony)
8 Cards: 48 VP
And I think even that is stretching the bounds of fitting the game well. The first two purchases are worth nothing in order to balance the insane strength of having lots of them.