Castle isn't a PG IMO. Best to leave him as SG. Haven't watched enough Vassell to say what he is. Fox can break down defenses but he'll need a team full of shooters like Boston. He'd be best fit there offense wise because that's what Celtics built with Kyrie and then with IT. Celtics still don't got a PG who breaks down defenses. But can't see them part with Holiday.

The point will be to surround him with a wealth of quality wings

This is obviously the trick though, the entire league is trying to do the same thing. Sometimes you have to work with what you can get. If the addition of Fox gets the team more wins, that makes SAS a more appealing destination for other players to want to come.