There is no written VMO/MMO. You just can't fly into the upper red bad that constantly fluctuates based on current flight conditions. The upper red band sits around .91/.92 usually. However you will struggle to get much more than .90 in cruise. Also you lose all the aircrafts efficiency above .87 because the cruise flaps lock out, however even with he cruise flaps locked out you will still save more fuel than a 767.