What do you mean by "accept Francis"?
The SSPX have always accepted that the Pope is the Pope. Archbishop Lefebvre always accepted the reigning pope as the pope.
Yes, but the hypothetical scenario was that every group *except* SSPX elects Vigano as Pope, while *only* the SSPX continues to recognize Francis. That seems really unlikely.
I think if all the reasonably solid bishops (and I mean everything from Burke or Sarah on the "left" to the Resistance on the "right") were to determine that Francis was a formal heretic, not really the Pope, and held an emergency conclave of some sort to elect Vigano, I don't think the SSPX would stand alone and be like "no, Francis is still Pope." I find that unlikely TBH.