I once did that route -HKG to SGN on a cx operated (vn code share) connecting to Cambodia Ankor air to REP, with a 1.5hr connection time (18months ago). It's sufficient time.

You go to a central transfer desk (it's signed) before immigration. Collect your new boarding pass. Then turn around for the door in the wall where you have an x-ray scanner for security. Up the stairs and your in the departures terminal.

Cambodia Angkor Air is alright - a novelty even. VN and Angkor Air change their schedules often (based on load I think) so just check your connection time before departure.