Is it confirmed that Southee is the vc for the test side, or is this still a rumour? If true, then that is just bizarre. Williamson was captain of the u19's (a side which featured several current BC's), of ND (a side which featured several current BC's) and briefly the NZ ODI side. I don't buy it being because Williamson's place in the side is uncertain. He just scored a 100 in his last match and has performed better than any other New Zealand number 3 since Fleming (not saying much, but it still counts). Even if his number 3 spot were in question (which it can't be any time soon, given the lack of viable replacements), Williamson would be more likely to drop down the order rather than drop out of the side altogether.
Bowlers almost never make good captains, and I've seen nothing from Southee in his career to date to suggest that he'd be likely to buck that trend. Maybe Hesson legitimately believes that Southee is captaincy material, but if this is true, it's very hard to see it as anything other than a calculated powerplay to entrench McCullum's position by filling the vc role with a supporter of his.