Thanx. I checked on Google Maps, and it now has Thailand's provinces, and Thai-alphabet versions of Thai place names.
Ubon Ratchthani is in eastern Thailand, and the closest sizable cities in Vietnam are Danang and Hue, across Laos.
As to Nong Khai (northeastern Thailand) - China, going northward would be about 400 km across rather mountainous terrain.
But going eastward from Nong Khai across Laos to Vinh, Vietnam would be easier - only about 130 km of mountains to cross.
Kanchanaburi is west of Bangkok, and the mountains there are further to the west.