After testing, it's actually possible to win the second UHV on normal speed, though you do need to be lucky with combat odds (I savescummed a bit, not as much as I expected - either you get momentum in the first turns of war or you'll fail, it feels like). I went:
1) Rome,
2) Austria (vassalized after taking Milan),
3) Spain (never vassalized even after taking all of their core cities - they gave San Juan in a peace treaty),
4) England (vassalized after taking London),
5) Prussia (vassalized after taking Frankfurt and Berlin).
Poland friendly-vassalized on its own, pushing me above 50%. I actually feared I wouldn't get the North America one.
I'll test another day if I can survive that absurd overexpansion score.