routes? Thailand, Cambodia, Vietnam

Hello, we are thinking of flying to Bangkok and on to Angkor Wat , then
taking a boat on to Ho Chi Min City. OR would it be better in the reverse?
We will need a visa to enter Vietnam and have the understanding that you
have to have it dated exactly for the date entered and exited. Is this true?
We have 2 1/2 months, so we just want to travel at our leisure. Any advise?
Thanks, Val

Hello, we are thinking of flying to Bangkok and on to Angkor Wat , then
taking a boat on to Ho Chi Min City. OR would it be better in the reverse?

Which time of year are you going in? Avoiding rainy seasons (which
hit different parts of the region at different times) should be your
main consideration.

Do you mean the boat from Siem Reap (the town nearest to Angkor Wat)
to Phnom Penh?