Am current on the same type as the Air Asia aircraft. VMO/MMO is only used to establish the rule distance (in our case, 1272nm for 180 min EDTO). It is NOT a requirement to fly at VMO/MMO during an EDTO diversion; furthermore, the newer types in the fleet (A350/777ER) are based on VMO - 10.
In addition, as VMO on the A330 is 330 KIAS, a diversion at 325-335 KIAS isn't doable. FWIW, our company policy is initially a driftown at Green Dot. Thereafter, flight beyond the nearest 'suitable airport' is only justifiable following consideration of numerous factors related to increased safety. Would have been hard-pressed justifying a decision to continue past Learmonth.
But I wasn't there, so I don't have all the facts.