MFS gives the story very succinctly (as one would expect) and the numbers are typical for turboprops ...
As a "for instance", if I look up, say, a KingAir 350 POH which happens to be on my desk just now, the relevant limits are
Vmo = 260 KCAS to FL210, then reducing to maintain
Mmo = 0.58 up to the max level (FL350)
The POH even belabours the point by stating that the "red pointer reflects Vmo/Mmo limits".