His point is that people trained to be experts in a field (vocation) are well equipped to do things and to give opinions about very specific points related to their vocation. But if you’re looking for a more comprehensive view, an overview suitable for national politics (“the plots and marshaling of affairs”), for example, it’s better to consult those with a broader education.

“Learned” in Bacon’s day would have meant someone schooled in ancient languages and history. I think he’s simply saying that men having that kind of training are better suited for politics than those trained (for example) as blacksmiths.