They are Partners in ASEAN and Friendly Competitors.
How they see each other? Mostly friendly and respectfully (except when on the hot football pitch).
I think Viets may detest Thais for a while as being the base to bomb Vietnam in the horrific wars. That is past. Viets may be a bit jealous of Thai economic growth for decades, never colonised and untouched by imperialistic wars unlike the Viets who suffer so much.
Association of Southeast Asian Nations - Wikipedia
Within ASEAN all are partners, and they trade. There is little quarrel except perhaps over a few border issues here and there which involves minority, But all is quiet and peaceful and the peoples are friendly. The idea of ASEAN has done so much to bring all of ASEAN into a harmonious consultative community, with each acting as mediator to defuse any issues through quiet dialogue. So we can say today, all in ASEAN are good friends and respectful to each other.
For most, since WW2, Vietnam war, the Khmer Rouge problem, Indochina seems quite peaceful and friendly, especially after ASEAN was formed.
WARS?? Someone wrote that Viet and Thais had no border hence no wars. Actually quite a bit.
But actually, wars in Indochina was very widespread across all countries. Burma, Thailand, Laos, and Cambodia and other ‘old’ kingdoms fought for thousands of years. The history of wars, allies, tributaries, vassalage, Mandala tradition, enslavement and conquest was common across all states and tribes.
This write-up on Laos gives an indication of the fights between all and makes for quite interesting reading, eg. just read Laos, which involves all countries.
History of Laos - Wikipedia
Below are some wars between Thailand and Vietnam.
Siamese–Vietnamese wars - Wikipedia
During Vietnam war, the whole of SEA especially Thailand was most edgy with the communist regimes in Vietnam (and Cambodia). Hence Thailand was far more pro-US than any other SEA country.
Still today, I reckon relationship between all are stable and normalised. We cannot underestimate the usefulness of ASEAN as an organisation as that helps a lot much like EU. There is a formal channel for dialogue and far more moderation and consultation unlike before. Many issues are sorted out behind the scene, and most problems are tempered and resolved amazingly well, including the decorum and good sense to dampen on nationalistic sentiments over disagreements. In China-ASEAN cooperation, the whole region works in unison to increase cooperation and trade with China, and pushing to formalise the RCEP trade agreement with China.
Overall, Viet and Thais respect each other, and the people are not too different from one another, although Thailand may be more indianised, whilst Vietnam is more sinicized. I believe Vietnam could have been a more prosperous nation if not for all the wars, and for the level of higher corruption.
For silk road, Vietnam has an advantage being closer to China, but Thailand has the advantage of being the natural hub for all roads/rail at the heart of Indochina, as well as having the Kral Canal (which will be likely be built one day).
I think the current worse fight between them is the fierce rivalry now in soccer. Thailand seems to have an edge for the time being as far as I can remember.