My thoughts also as of about two weeks ago:
Cain/Gingrich or Gingrich/Cain.......
but I also can’t see Gingrich settling for Veep.
And you can just anticipate what ‘some’ people would
make of Cain occupying the “secondary” slot.
Especially if polling still shows him ascending past
Gingrich months from now.
It's not like "somebody" would decide the order, should that ticket arise.
The top of the ticket will be the one who wins the primaries and selects their running mate.
Any other poll has no meaning at all.