Whereas the Vandals and Byzantines are probably the easiest way, the Normans could still introduce a romance language. If they bring in Italian settlers, yet their North african kingdom separates early from their Italian possessions, and then traders from spain and southern France add into that mix we could get a separate language by now.
The problem in all scenarios is how to let the language survive against Arabian. To keep the romance speaking areas out of Arabian rule for centuries, I'd say that Egypt would be key. If somehow the Byzantines manage to hold Egypt, Egypt becomes an independent Christian state or the crusaders conquer Egypt and somehow hold it/the Copts take over, then the North african romance speaking country has only to defend against other European, and hence probably romance speaking invaders, and Muslim invaders from the west.
The problem in all scenarios is how to let the language survive against Arabian. To keep the romance speaking areas out of Arabian rule for centuries, I'd say that Egypt would be key. If somehow the Byzantines manage to hold Egypt, Egypt becomes an independent Christian state or the crusaders conquer Egypt and somehow hold it/the Copts take over, then the North african romance speaking country has only to defend against other European, and hence probably romance speaking invaders, and Muslim invaders from the west.