Belarus authoritarian leader has chosen close alliance with Russia, but denying Belarus statehood is just silly.
That's what the term "vassal state" means. It doesn't "deny statehood", technically. What it indicates is that in practical terms, a state (or the regime running it) can become so dependent on the resources of a superior state (indeed owing its very existence to it) -- that it is fair to say that it is independent in name only.
Which pretty much describes the relationship of Belarus vis-a-vis Russia, after the events of 2020.