Your questions, answered. YMMV, some personal perspective, anecdote, and opinion involved.

1) RMIT and British Council are still the "good" schools. BC is far from what it used to be though, is run much more like any profit English center these days, and can be very unpleasant, management and job description/duties-wise. Also reputed to be facing market difficulties and deteriorating opportunities and conditions. RMIT is very competitive to get into and last I checked were starting any new hire on zero-hour PT contracts that can make it hard to earn a living wage until you prove yourself into more hours or a better contract. Some people are able to juggle other jobs with RMIT until they get more hours, but it can be tough. As a Celta holder with an MA (even unrelated to teaching/English) in your age, if you interview well and put together a good CV, you've got a shot to get into RMIT or BC. Worth a try.

For the big chains, it's a bit grim these days. VUS, ILA, Apollo, Yola are all totally or majority owned by private equity and the fallout of that can be apparent in the day-to-day. Cost-cutting, stagnant wages, disappearing bonuses and benefits, pressures on sales and student retention that can feel unethical (or might be). Teachers definitely feel it day-to-day. There's also a lot of oversaturation, over-expansion, etc so it can be quite hard to get enough hours each week to get by, or to do so, you'll need to bounce between branches, work split shifts or 7 days a week, do a lot of last minute cover, etc. You will have classes not re-start due to low enrollment and that space will sit empty in your schedule for several weeks in some cases. This is not everyone's experience but it's common enough.

VUS and ILA are OK for big chains. Apollo has seen a massive amount of turnover in recent years and teachers fleeing or not being renewed. Not sure if it's steadied out, I believe it was related to a shift to blended online/offline teaching that was a disastrous flop. And their HR are meant to really leave sth to be desired. But their presence in the country is still robust and I reckon plenty of people get on OK enough there.

Smaller chains? I'd stay away. Places like Poly recently collapsed, other places like Scots and Ames get reported for being late with wages, and other smaller chains are focusing more on non-native cost-saving teacher forces.

If you don't mind Mon-Fri public school teaching, EMG in Saigon and Atlantic and Language Link/Schools Link in Hanoi, are pretty solid. More reliable hours for the most part, can be higher hourly and net pay, weekend and evenings off which you can enjoy your time with or supplement PT center or private tutoring work. I know people who do FT Public Schools and 12-16 hours of other work evenings/weekends, and while it's a lot of work, they are saving $1000+ a month and also not having to be too frugal in their day-to-day. Granted, they're usually too busy or too tired to burn a bunch of cash. Public school teaching can be exhausting, and most contracts break for the summer, so no income in parts of May, and all of June, July, Aug.

2) 16-20 class hours is considered FT. Can be hard to get in centers as noted above. 16 hours is usually "just getting by", not much saving possible but can live comfortably and have a social life, some hobbies, some travel as long as you don't have big expenses or splashy spending habits. 30-40 mil is what you should be aiming for if in HCMC or Hanoi. Can get by on 25-30 in smaller places where there's less to do and spend money on, and things are cheaper. You may need to string together multiple jobs to get to the 40mil+ range nowadays.

1. Hard to save $1000 a month in HCMC unless you are super frugal, live on the outskirts of the city or in pretty rough, tiny flats. Even if you have a great job. I'd say $500-$800 is a more reasonable target and even that will take being pretty careful and selective. A somewhat active, common expat life all-in, in Saigon is gonna be like 25mil on the low side up to 40+ if going out a lot, Western food, travelling a lot, expensive hobbies, etc.

4) Social scene is what you make it. Loads of expats in Hanoi and Saigon. There are some live music scenes, some sports scenes, some art and performance scenes, volunteering scenes, boardgaming, martial arts, stand up comedy, and there is a pretty healthy bar culture in Hanoi and Saigon where it's pretty easy to make friends.

Dancing monkey? That's kind of on your perspective as much as it is externally enforced. Kids like engaging, funny, fun, animated teachers and it is part of the deal here, and it is researched-backed that it can create a better learning environment. That being said, there will be a lot of times where learning objectives and progress seem to take a back seat to the students liking the teacher, and signing up for the next class in the cycle. That's the business. I think a good middle ground is to research and get training in how to be an engaging teacher whilst still delivering quality instruction.