I think of it less as growing a spine and more as realising they might be heading to a situation in which they have no choice, so better to get out ahead. It's obvious that the Vance team (whoever that might be) has cultivated good leaking relations with the press - I think Stephen Miller is fairly tight with Vance, and he seems to have his hands all over everything.
@AcrossthePond55 Re 2028. Maybe. But I think he knows his best chance for 2028 is by succession rather than election. Trump seems to be favouring anointing Rubio, and Vance as VP gets less and less popular by the day. But if he's in office he can exert control instead of having to dance to the whims of the unstable despot. Loathsome as he is, if he can appear to bring back some stability, I suspect a lot of people will be more inclined to give him the benefit of the doubt in 2028.
Anyway, just a theory. I could be totally wrong.