So, I will talk about commissars in another army that still fields them and take inspiration from the two you mentioned: the PAVN. Now excuse me if I am being vague but there is a thing called the cybersecurity law and the last thing I want is for my arse to land in the slammer for a few karma. But I am a whore for karma so here goes.
Commissars in the Vietnamese military is extremely important. A funny thing is that while they are army officers, the army has no say over them. That is because they belong to a parallel formation: the Central Military Commission or CMC. The CMC is under direct control of the Communist party. On paper, the Dept of Defense (DoD) is under the control of both the government and the party but since the party is in control of the government (being the sole party allowed to rule based on constitution), the army is basically commanded by the Party. And I will not open a can of worm about "who controls the army" because opening that can of worms will get me into some hot water.
So on paper, the CMC is only supposed to manage the General Department of Politics. In practice, through the DoD, they control everything. And Commissars are how they exert control. And boy oh boy there are a lot of them: according to regulation 59-QD/TW, there is 3-5 commissars and 9 party members (who are, supposedly, commissars assistant/commissars in waiting) for every company. Meaning that you can have 1-2 commissars for every platoon.
Now, before I talk about what the commissars actually do, I do want to note one thing: the things that is written on paper is different from what is actually practiced in real life and nothing is ever recorded and that recorded this is most often time BS. This is ever more true in Vietnam where everything is a shade of grey and power overlaps each other.
To quote regulation 59 again, "the commander and the commissars must follow the every single aspect of the orders given to them by higher party committee". This basically means that whatever the higher party committee, which is made up of higher-level commissar, order, the lower obeys. The commissars will see to it that the party's will is observed and any commanders having problems with it will have to have a long, hard talk with their commissars. Now it does not say explicitly if an officer has to ask his commissar permission for anything, but it is written clearly in the regulation that whatever he does he must inform to the commissars.
The second task of a commissar is "political struggle" aka making sure that his men is motivated to fight and is loyal to the Party (as the slogan said "Loyal to the party, fillial to the people". I am not making this shit up btw here is the source by the military mouthpiece itself https://www.qdnd.vn/ho-so-su-kien/loi-bac-day-nam-xua/quan-doi-ta-trung-voi-dang-hieu-voi-dan-558702). By how ? They don't hold feed soldiers, don't deliver mail, don't evacuate their soldiers to reply to DuncanM's question. Those are dealt with by the professional. The commissars are there to make sure that the soldier's mind remain "pure" and they won't be reading any subversive stuff any time soon. They will often do this by holding self-criticism section, organizing newspaper reading each night where the Party mouthpiece is read out loud and they will pretend to listen to soldiers' complain before going on to ignore them all, may be throwing a party or some kind of party-sponsored activity as a way to instill some love for the Party in the men.
And on the big question: do they shoot their own men ?
The answer is: no
It is this sensitive topic where everybody says it did and will happen and yet nobody got proof other than words of mouth and I-hear-Nguyen-from-next-company-said-so. Most of the soldiers I talk to all tells me that commissars are there, are ready and willing and able to shoot you if you show any signs of cowadice. One common story I heard dated back to Vietnam war when a lot of young soldiers were punished with some shot. Why ? Because they had a white handkerchief with them. Silk and garment was very valuable back then and the color white was treated as luxurious. So loved ones would send gifts of white handkerchief to their sons. Problem was, white garment could be used to signal surrender and many men got shot for that. How true is this, I have no idea. Nor have I managed to find any definitive proof on it. So I will have to go with a no.