I mean, thinking about it a bit more, there is a plausible scenario where Rubio could be Vance's VP and then run himself.
Technically, VP is a step up from Secretary of State. But Rubio was a Senator for 14 years vs Vance's 2. He was in the State House of Florida when Vance was in high school. It's not unheard of for a VP candidate to be older than their President, and even to successfully run afterwards (LBJ, Biden). But it is highly irregular. Imagine if Hilary had just agreed to be Biden's VP after being Senator, Secretary of State, and Presidential candidate. It looks like a step down if you have that much experience and settle for anything less than the big chair.
Rubio would be 65 in 2036. That's also a long time to be riding on the coattails of other Presidents and hoping they have wildly successful terms that don't drag you down. A major crisis, bad war, economic collapse, or scandal in a Vance Presidency as late as 2035 could nuke his chances of running and winning in 2036.
For either of these guys, the best scenario is one agrees to step aside and support the other, and be waiting in the wings for 2032 if the guy who runs in 2028 loses. Other than that it's speeches and book tours til they die. Such is our political system.