I think pretty much the only similarity between herald and vassal is how with both you really want very high action density decks. And while both are strong in the right conditions, Iīd say herald is a lot better card. Due to a few reasons
While +coins is important, itīs +cards and +actions, that actually allow us to do crazy stuff. A herald based deck can look at the 9(usually) other action cards in the kingdom and figure out whatīs the best payload. Vassal needs to look at the other 9 kingdom cards and figure out how to build an engine with them. (And thatīs a major point with vassal, itīs definetely an engine card, but it doesnīt actually help much in making the engine work)
Both herald decks and vassal decks will mostly need trashing, but the first will generally be looking for a +coins action card while the second needs village support, my point here is not that the former is more common than the latter (which I believe is true, but I really have no data to back this statement), but that in the later you actually need a village in hand before you can safely play the vassal, this means a vassal based deck is much more prone to duds than a herald based. This also leads to my last point
Vassal fits in a rather clunky space of Dominion cards that Iīll call maybe-terminals, itīs a card that when you play it you donīt know if itīs gonna be a terminal or not (If I recall correctly the only other cards that are like this are Golem and Tribute, but I may be forgetting someone). That can lead to very diffcult decisions where you have a vassal and another terminal and donīt know which to play, itīs the weirdest thing in the card for me...
Well... That said, I donīt think Vassal is a weak card, itīs very interesting engine payload, you just shouldnīt expect there to be vassal decks in the same way that we see herald decks...