You can star from Hanoi go to Halong, Sapa then fly to danang, hoian .... bus/train to Nhatrang, muine, Hochiminh City. From Hochiminh city you easy to book open tour to mekong delta and Phuquoc island.
From mekong (chaudoc) you can link to PhnomPenh Cambodia by boat (speed boat 4rs slow boat 6 hrs) or you go to by open bus from HCM (6 hrs) many bus most of them star from 8am :-)
Phnompenh go to Siemreap by bus/boat . If you want to go to Bangkok by bus from Siemreap that will be amazing (SR 8am - bangkok 5pm) :-)
Good trip Kent !