a) With all the autonomy he's had as SoS, and even though he's also been a genuinely loyal team player in the Obama administration,I think he'd chafe being in the much less autonomous/more subservient position of VP.
b) Also, I don't see him as a close working partner of either Clinton or Sanders.
c) Also, I think the Democratic nominee is going to have to have a running mate who's a little younger.
Mostly, I just want JK to still be out there doing good, in whatever capacity he can be most effective. I don't think that VP is that job.
While hanging on as SoS for a while may be an option while the next SoS is confirmed, I don't see him (see above) as a compatible partner with either Clinton or Sanders. Too bad, because he's an outstanding Sec of State, easily the best in my lifetime.But I think that's the political reality.
Maybe a special envoy of some sort?
I'm sure (and certainly I fervently hope) that he will manage to keep himself out there, doing great and courageous things.
Also, I'm really looking forward to his memoirs, and I hope that those memoirs will be both thoughtful and as frank as humanely possible. I'm sure that he has LOTS of interesting stories to tell, especially about the 2004 election 
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