Vmo relates to pressure loads on the structure. While nothing is going to happen a modest margin above Vmo, there comes a point where bits can break off .. eg wing LE D-cell skins.
If I recall correctly, the RAAF P3 accident which ended in the lagoon was an undesired consequence of a pass at (an unintentionally) too high an EAS which saw some tin cease to maintain company with the parent structure.
Similarly (and the memory is a bit scratchy here) I recall the early V-tail Bonanzas had a similar problem at lower than expected EAS due to inadequate structure margins in the tail nose ? Hence a mod to strengthen the region after a number of inflight problems.
Vmo is an EAS limit so you would normally expect to see an AFM limitation in IAS (CAS) either simplified to constant, or increasing modestly with altitude.
Mmo is a sloping line from high EAS (at low level) to low EAS (at high level). The two lines cross somewhere on the way up .. typically in the high 20s. At lower levels, you run into the EAS limit first .. at higher levels, the compressibility limit.