I generally value 1 less card at a "1 less VP hit". Here, I'll trash Provinces if it's early game via Bishop, as 5 VP tokens is close enough to the 6pts the Province was worth. However, I only do this if I have nothing better to trash. I still go for Estates, Shelters, and Coppers first. If there's a higher cost card that's no longer useful (like Young Witch, Sea Hag, or Potion I suppose), then may as well clear that up while converting it to VP. If it's later in the game, I won't really do this, as the impact on my deck is getting to be negligible.
If Goons, I'll often buy a Copper if it means getting 2 VP per buy. Of course, not all games have branch actions, and using TR or Kings Court doesn't multiply the VP tokens you get per buy.
I agree with the post on Monument... it's more so a supplemental, as I'm sure your opponents will be able to keep pace by buying the higher scoring cards (esp. if it's a P/C game... 10pts per Colony!), or integrating Monuments into their own strategy.
Another nice thing is sometimes the VP is guaranteed, whereas in order to buy the Victory card, that's luck of what I end up drawing.
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And then Empires is a whole can of worms on its own. I bring this up b/c I believe the better way to go about this is to address each card/Event/Landmark, covering a few interactions/synergies where appropriate.
Also brought up in another post, it can be worth buying a Copper if it gets you 2 VP.
Other cases... instead of buying a Province, buy a Gold for $8 so you can trigger 2 VP off Basilica. The Gold is sort of like a souped up Harem (whereas that was 2 VP and produces 2 coins, this produces 3 coins and is "worth 2 VP"). Ditto with Arena... I'd rather not draw into dead cards. Just discard it for 2 VP. Battlefield has gotten me to buy Victory cards like Estates and Duchies earlier than not as the extra 2 VP per each of them does provide more bang for the buck.
For $7, a Wedding is nicer than a Duchy early on for sure. Later, it can be nice to buy it if the Duchies are out and alternatives to getting pts aren't that great.