I more or less concur with everything said so far. You only really want it in very action-dense engines, which requires good trashing, which means you should get a trasher early. Opening Vassal seems a little odd.
In those engines, it adds "+$2" to the next card on your deck (assuming it's an action). It has very nice synergy with deck inspection, but building a Sentry/Vassal/maybe-also-Harbinger engine sounds a bit slow and very wonky. In a more vanilla Village/Smithy engine it probably has a place; it's nice but not super-OMG-awesome. It's a payload silver that most often will (effectively) give back the card and action you spent to play it; sort of like Conspirator. If the engine draws deck, you'll want Vassal early in your turn (so you're not playing the top of an empty deck) where you normally want Silver late (because you want Village/Smithy early so you can continue drawing cards). Although: if you have 5 Vassals in hand and 4 cards left in your deck, discard 4 vassals to Cellar, then play Vassal to make a big Vassal chain, woop! ;-)
1. If there are not other terminal actions that you plan to buy early on, then might as well take Vassal over Silver
Edge case: you want to play Hunting Grounds/BM, or BM with some other expensive terminal draw: you probably won't buy it until late in the opening or in the early mid-game (which for some value of ... is not "early on"), but once you do you definitely want to have silver over vassal. More clearly stated: whether you want Vassal early depends in part on which terminals you want
late(r).
Opening Vassal doesn't seems appropriate when there's an engine but no trashing, because then you don't want Vassal at all. It also doesn't seem good in expensive terminal draw/BM, where you want a Silver. It doesn't go into a Workshop/Gardens rush; so it only really seems good in something that smells a lot like BMU. And in that case, you don't probably want Vassal at all, definitely at most one, because the risk of collision vastly outstrips the chance of having one vassal in hand plus one on top of your deck, and the infrequent upside seems like it shouldn't outweigh the frequent upside; just buy silver and gold, man. (Okay, maybe the increased cycling of a single Vassal helps enough in the beginning to outweigh the badness of cycling late, while greening, but that margin is probably
very slim; it's probably positive, because you cycle a larger fraction of your deck early, maybe?)
The strongest case for
opening Vassal is probably the wonky Sentry/Vassal/cantrip-heavy engine thing; open Silver/Vassal, then get two or maybe even three Sentries ASAP to get thin; the +$2 helps you get to $5
and you also want it later on; also, you won't ever draw the one Vassal dead when all other actions are cantrips. Once you get thin, add more Vassals. Until then, add other cantrips, e.g. some of Harbinger, Merchant, Poacher, Market. Maybe. I think it has to be a weird board for that kind of strategy to be dominant, but hey, weird board sometimes come up, especially in base set only. I remember a game where I played a Sentry/Market/Merchant deck and it worked decently; one or two Vassals probably would've been a fine addition to that deck.