Wouldn't nationalist feelings make this unlikely or would back the Thai due to anti-communism
Firstly, it depends how and when the Thais intervene in each respective country. With Cambodia they will overrun the western and north western provinces stopping at their respective provincial borders, after all the predations of the Khmer Rouge was known even at this stage due to the defection of a Khmer Rouge Battalion to the Khmer Republic in 1974. Hell the number of Cambodians that sought refuge at the French embassy following the fall of Phnom Penh supports the idea that people suspect what their fate will be.
Back to your question - the Royal Thai Army can use units that are primarily comprised of ethnic Khmers to stabilise the respective provinces, these provinces then declare their independence from Cambodia and this is recognised by Thailand. Over time as Cambodia degenerates into a dystopia, a vote could be held to unite these independence provinces with Thailand. Considering the refugees that would be fleeing into these provinces then it is likely that a vote will legitimately pass.
In Laos the Thais will conquer the provinces nearest to their borders, but would not attempt to conquer all of Laos due to logistical constraints and the presence of main force PAVN units.
With communist guerrillas or conventional forces and would this include Vietnam?
In 1975 in Cambodia the RTA will initially encounter scattered Khmer Rouge units, however within 3 - 6 months they will face a conventional offensive by the Khmer Rouge to oust them from the provinces. My assessment is that the Thais would defeat them, particularly if it is a conventional battle, however it will probably result in a drawn out frozen conflict with both sides raiding the other.
Laos they will fight the Pathet Lao quite quickly, but unless they push east are unlikely to face PAVN forces. The question is what will the PAVN do in this scenario, they will either stay in place or they will support their allies and attempt to drive the Thais back, which has the effect of postponing the Easter Offensive to probably 1976.