"Even if we say that correlation does equal causation (which it doesn't)"
If causation equalled correlation, then there would be a 66% chance one of your guesses was correct.
But keep in mind that if you're saying that the vaccine lead to higher deaths, and lockdowns (and the pandemic stress in general) lead to higher deaths, and one of those became true, then you're still only 50% right and 50% wrong.
If I was betting on it, then I would guess its a combination of stress from lockdowns and pandemic (plus sedentary lifestyle from lockdowns), plus Covid-19 damage it has done or the bodies (that had been heavily researched)
But I would say that stress and lack of exercise, plus excessive drinking during lockdowns, is the higher cause.