@Larry ****
Strangely enough I found Vietnam to be a lot cooler than Thailand at the same time of the year. Depending where you are in Vietnam, the further north you go, the cooler it gets. Also as Vietnam has a long coastline bordering the South China Sea, it has the benefit of cooling sea breezes which Thailand seems to lack., especially during that hot Songkran period starting April then May/June. Cambodia, being almost landlocked, except for Sihanoukville in the south suffers from the same heat issues as Thailand. I am guessing here, but most likely you will be going to Angkor Wat (Siem Reap) in the north -west of Cambodia, so again bloody hot in that April/May/June months.