One silver lining to that is no matter when Vance takes over, if Trump dies during his term it counts as a full term for Vance, so he’d only be eligible for one more afterwards.
AIUI, no.
No person shall be elected to the office of the President more than twice, and no person who has held the office of President, or acted as President, for more than two years of a term to which some other person was elected President shall be elected to the office of the President more than once.
It is totally dependant upon when they have to take over, without considering the timewise state of the office-holder they take over from.
No matter when Vance gets the position during the next stint (or before, if it's forced to happen prior to Inaugoration?), he'll get the opportunity to run in his own right in 2028. If such a takeover is in 2027+ (or if '28 is another Grover-like skip term due to failure/non-standing), there's another possible (re-)election for him in 2032. Could be up to (but not beyond) 10 years of POTUSsing.
Not that I think it matters. There'll be Vance-like people ready to step up if Vance is unable or unwilling to continue, if not beyond his level, if his side of the political spectrum continues to move over and there's no effective anchor/counter-pull. And four years could well be an eon in politics, both local and world.
Personally I hated Nancy too, for her trip to Taiwan few years ago, she nearly triggered a war that summer , she’s an ugly and disgusting warmonger from what she had done…
With respect (and I mean that, it's no insincere platitude) whatever we think of Pelosi[1] she's not the one regularly running intrusive and aggressive military exercises against Taiwan. Her going there might or might not be wise, but it's hardly a casus beli in the face of the continuing provocations from "the mainland". At worst, it was posturing, but the manufactured rage didn't rely upon it.
But I'm mindful that you're going to hear different things on your side of the Great Firewall. I've had conversations with those in China who assumed I knew all about random exam scandals, but didn't know much about the incidents around the Spratly Islands/etc.
[1] Not sure I like her, and whatever political skeletons there are in the cupboard, but there's more integrity there than in certain quarters I could name.