I'd go for option "B". There is nothing that looks quite as tacky as two different sheet vinyl floors meeting, threshold or not.
Have you looked at composite wood planks, Pergo type of thing? I installed one of the cheaper Lumber Liquidator composite planks with a wood grain in my trailer five years ago. It is pretty much just like the day I installed it, no buckling, separation, or other nonsense. Some of it is covered by throw rugs which are a lot easier to maintain than pile carpeting. An owner would have to know how slides are operating and if they touch the existing floor, clearance issues can be a headache.