I spend around 5 or 6 months of the year in Thailand. Every morning when there I have an electrolyte drink, a sachet (there are several brands) that you can buy - usually between Thb 5 to 8 per sachet. They help to combat dehydration and help to replace minerals lost through sweating. Take with water.

No idea what a pro-biotic is, sorry. Sounds like marketing hype?

Never taken Pro-Biotics and rarely get ill, once in India and once in Myanmar.

Tips you should plan your route. I suggest Thailand first then Vietnam then Bali. It is the start of tge rainy season in Thailand in May. June should ok in Vietnam although getting hot. Bali is dry season, May-September. You Could sway Vietnam and Thailand around, don't think will make that much difference, weatherwise.

16 days is not long for Thailand or Vietnam, 13 days for Bali maybe too long? Bali is quite a small island, the beaches are not that good.

Don't try to do too much or you will spend a lot of the time travelling between places and you may get sick of airports. Can take most of day between places.

Given that the trip is less than a month away, I presume flights are already booked, so a change of schedule is unlikely. This has also been posted on Bali and Vn forums, so plenty of other advice already.

I'm with Ray on don't try to do too much. It's a common mistake and apart from a tick on the been there seen that list you spend lots of time travelling, little time at the sights and a very frenetic trip. It might seem like a long way away or the trip of a lifetime, but in reality all those countries are only 12 hours-ish away and can be visited again if you like what you see.

Why wait till you go abroad to improve your gut health. My advice is to eat as varied a diet as possible. Lots of veg, fruit, nuts, etc. Anything to improve your guts and get it used to different food types. Chili's may be a useful addition. I'm always sceptical of commercial 'improvers' as they often contain lots of unnecessary additives to flavour or preserve. They may also temporarily upset your stomach.

If there was a magic potion to prevent illness then someone would be very rich.

I would google how to stay healthy abroad or similar not for products but for basic guidelines and I would ensure my vaccinations are up to date. Many people forget that in SEA western food items such as mayonnaise can be a problem

Hey, we travel like your plans regularly and I would firstly say that honestly we found Thailand to be considerably the best country in SEA to travel at the moment. We found the environment to be cleaner in the tourist spots as well as in cities. Also the quality of food better and clener. We had no issues on our last 10week trip around SEA including 7 weeks in Thailand. Perhaps we were lucky although we were more aware of what we ate. We ate in street food type places in Thailand but found those places too grubby in the other SEA countries we visited. It is worth taking probiotics I think starting before you go. The other option is to eat organic an apple most days including the core which is full of probiotics. (but not the seeds), and make some effort to eat healthily before you go. Avoid over processed fast food in the airports.

We take a filter bottle (eg Fill2pure) and filter tap water, saves wasting so many plastic bottles.

It is worth doing a bit of research on antibiotics in case you do get a bacterial infection (eg dysentery through food)

TBH with 7 weeks I am tempted to suggest Thailand only as it has such a variety of history, beaches (way the best), culture, and the most fun. Bkk then South then North Is a long way with so many fabulous stops with nice rooms and brekkie for around 1000-1500 baht even in beach areas.

Happy planning