I've thought for some time that Biden would be replaced on the ticket with 90 days to go. We're a bit past that, but I still think Biden will go. Team O won't even have to mention any of Joe's screw-ups if they replace him. By now, Biden is a joke, and many people are wondering if he is becoming senile. This latest absurd "chains" comment will get so much attention that it will be what the talking heads cite as the last straw.
As for Biden's replacement, there probably aren't any viable hispanic or black VP choices for the Dems, so they will probably go with a woman. O has nothing to gain from Biden anymore, and will probably seek to strengthen his take of the female vote, which Romney/Ryan would erode over the course of the campaign.