I'm embarrassed I don't know, but if Romney can get no running mate?

That is really pretty impossible, but assuming it actually happened:

A party nominates a candidate for President and one for Vice President. State laws dictate how someone gets onto the ballot, but the major parties operate in all 50 States and are always able to get their nominees on the ballot. That's one of the benefits of being a major party nominee instead of a third party where you have to build the infrastructure all over the country to get on the ballot everywhere.

A candidate can be endorsed by a party they don't support, but most parties, and especially the major parties, have rules that require the candidates for President and Vice President to accept their nomination. Also, in most parties, especially the major parties, the Presidential nominee can choose the Vice Presidential nominee (assuming the VP nominee accepts).

The State ballots don't actually elect the President and the Vice President, they elect Electors appointed by the nominees. So the Republicans appoint slates of electors in each State. In any State where Mitt Romney wins, those electors get sent to the Electoral college. In the States where Obama wins, Obama's electors get sent to the electoral college.

The Electoral college then votes separately for the President and the Vice President and both must get a majority to win (or else it goes to Congress for a more complicated procedure). If there were no candidate willing to accept the nomination for VP, the electors would decide. If nobody got a majority, then it would be up to Congress. The members of the House of Representative would vote on the top two candidates, Joe Biden and whoever among the possible Republicans gets the most votes by the electors. The Representatives vote as States, so if a majority of the members from New York vote for Biden, that is one vote for Biden and if the majority of members of Delaware vote for a Republican, that is one vote for a Republican.

If whoever gets appointed by the House doesn't accept the appointment, then the office would be vacant. The new President would nominate someone and the Senate would vote whether or not to confirm that person. I suppose technically, the office could remain vacant for the entire term. In that case, if anything happened to the President, the Speaker of the House of Representatives would become President and if there were a tie vote in the Senate, the vote would not pass. Those are the only two duties of a Vice President, so there would be no other consequences of having the office vacant.

But again, that is a completely implausible scenario.

EDIT: People always say they don't want to be VP. Some have been a little more forceful than usual, but when actually offered, it is pretty hard to decline. There have also been several who have not declined, but rather deflected or said that if offered, they would have to consider it.

They will find someone. Romney is not such a bad choice actually. Right now, I am a little towards the Obama side, but there are things that could switch my stance to Romney very easily. I try to pick whoever I feel will do the best job and the party hoo ha nonsense aside, but Obama, for me, is getting a little too sure of himself. I think he may be in for a larger battle than he realizes. I could be wrong.

Romney has them lined up at the back door wanting to be vice president,all of them against gay marriages,born in the usa ,and have valid birth certificates,all of them are as good or better than any vice president Obama has,Romney will chose when he wants to,the democrats are waiting to dig up dirt on any one romney chooses,democrats are a sleazy bunch,that's why they picked Obama ,he is just the same as the democrat sleaze,

Hank Williams Jr. could have been a super %.. He might say the failings that Mitt needs to assert yet will no longer be able to (no longer yet besides). Hank might attraction to the single tooth, one branch kin tree base voter or different words the Sarah Palin fan base.

Oh, there's always some greedy conservative knucklehead ready to step into the limelight.

LMAO, You're doing a TERRIBLE acting job as a Republican

You can't be this naive. Can you?