I think it would look highly improper for the Senate to appoint an opposite-party VP, since it would in effect be an open invitation to assassinate the POTUS. I suspect you'd either have Ryan being appointed with Democrats voting for him or abstaining, or maybe, if Romney loses the popular vote and Democrats are particularly bitter, a compromise candidate acceptable to both sides (perhaps Colin Powell, Joe Lieberman, Dick Lugar etc.).
Aren't contingent elections for Vice President limited to the top two voted for in the Electoral College? If the compromise candidate was chosen before the Electoral College voted then it could work--in which political reality it seems like it might be easier to just have the Electoral College elect the compromise candidate outright--but otherwise it would have to be Biden or Ryan.
