At the moment, vacation in Vietnam is cheaper than in Thailand, but everything depends, of course, on the resort and hotel stars.

You can rest in Vietnam in June, but it's better to choose the resorts of Nha Trang and Hoi An - at this time in these resorts the dry season

Prices for these directions are similar, in Thailand there is more choice of hotels

In Vietnam in June you can and need to rest. I recommend Danang and Hoian