I don't mean to be glib or insensitive with this comparison, but it is hard to get out of my head and is a conclusion arrived at not just by myself but by many others who I've chatted with about Vampire Survivors: this thing is freebased cocaine. It is pure Skinner Box madness: sensory overload and number-go-up and metagame-y progression and all the things that make you want to play just one more round for a solid four hour block. I found the sensation of putting Vampire Survivors down not at all unlike the sense of come-down I get from modafinil; a deadening of senses.
All of which to say it is extremely fun to play, even if I didn't feel particularly good about playing it. The difficulty curve is kind of staccatto; once you have a good sense of the combination of various weapons and abilities, you can pretty much steamroll the game and the 'fun' comes in customizing your loadout so as to optimize for progression by getting as many unlocks with each run.
What keeps me from giving this a fifth star is that the game, while extremely fun, feels... shallow in a way that leaves one a little bit unsatisfied. There are two ways I think that a roguelike game like this goes from "fun" to "great":
Vampire Survivors certainly doesn't have the former, and not enough of the latter. There are a couple interesting feints at diegesis in the final few acts of the game (some legitimately "what the hell just happened?" moments, which are amongst the best feelings in gaming) but the game just doesn't quite play at that level.
If this review sounds critical, let me remedy that: buy this game. It is like four dollars and is tremendously fun and I am so glad to have played it.