Plausibility Check: Sub-Roman Tingis.

How possible would it have been for a reasonably large Roman population to have remained in Tingis after the Vandals invaded Africa?

Very possible. There was enough of an urban Roman population for Berber petty chieftains to move in and declare themselves "Roman kings." Unsure about Mauretania but in Tunisia, a Romance dialect did last until the 12 century even centuries after the initial Arab invasions.
 
It's quite feasible for sure. Eventually the Roman population would be Berberized, but not before a fair amount of Roman influence spreads to the Berbers.
 
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