In reverse order …
It used to be. Whether any of that professionalism remains today is very much unclear to me.
That is certainly a good guess if you / they want to go hardcore Fascist - trumpist.
Explanation please …
That’s now a rhetorical question by me. Some quick Googling provides an answer …
Google’s first cite (of two) is to this very insightful website loaded w the latest of political insider info: 
heh!
I disagree w this analysis. The President candidate is chosen primarily based on ability to win, and only secondarily, at best, on ability to govern. See trump, donald j fuckstick as Exhibit 1.
The VP is simply chosen to (incrementally) improve the electability of the combined ticket, and oh by the way meet the most basic criteria of possible contingent president-ness: be a citizen and have a pulse.
If Vance is replaced that solely speaks to his perceived electability, not his perceived competence.
Further all the above assumes a competent RNC insider cadre capable of rational calculation in battle w the Ds rather than being utterly consumed by settling internal vendettas by knife as I suggested earlier. Or simply by mutual leopard face eating.