Well, it's been a while since I went to Vietnam (2005), but, as I live in Japan most of my travels over the past 18 years have been in SE Asia for weeks (up to a month) at a time (usually once a year).
1. In the past I would take out all of my 35mm film from their containers, and pack them into ziploc bags. Yes, the containers are (mostly) watertight, but you can pack more film without them. Now, since I mostly shoot 120, I keep that film in it's individual packaging, and 35mm in their containers, just because I carry less of it. Everything still goes in ziploc bags, and I usually carry some desiccant to throw in there too. (I also keep some in my camera bag to help out the cameras and lenses too). If you are staying in places that have a refrigerator, you can keep your film in there to help keep things cool, and only bring with you what you expect to shoot during the day. If I thought I might be doing something water-related, where I would have my camera and/or film with me, I also pack a rollable dry-pack just in case. But in fact, I have never needed it. How are you carrying your gear? No matter how you store your film I would recommend some sort of rain cover for your bag in case you get caught in the rain - bags can take a while to dry out.
2. Sorry, can't help you with this one, in terms of shops that you can buy film at. That said, 2 years ago I went to Europe for 5 weeks with over 120 rolls of film in my carry-on bag (plus camera gear) and I still needed to shop around for film. Even though I was able to find some in all the major cities that I visited, it still took time out to find the various places that sold it, and even then, they didn't necessarily have what I wanted specifically (although I was able to make do with substitutions). Six years ago I was in Chiang Mai but there was no place for me to get film there, so I ended up having some shipped to me from Japan. Anyway, to make a long story short, bring as much as you can unless you know for certain there's a place that you can stock up on.
3. I can't suggest anything specific, but I hope you have a chance to visit smaller towns and cities and get out into the countryside. When I went there we visited Hanoi, Ha Long Bay, Hue, Hoi An, and HCM City. Ha Long Bay was a big tourist trap and very disappointing (the weather was also not good at this time of year for classic photos of the bay). I loved Hanoi, Hue, and Hoi An. We did a motorcycle tour into the countryside around Hue which was probably the best part of the trip. Most of my "classic" shots were from Hanoi and Hoi An. HCM City was just a big faceless city to me, without the charm or character of the other places that we visited. I also recommend getting up early. Early early. Sun is rising early - you'll see most Vietnamese out and about going for walks or exercising or getting ready for the day, without a single tourist in sight. I had a lot of interesting experiences with locals getting up early like they do and witnessing this part of the day.
4. You could probably FedEx or UPS your film back home if it becomes a problem schlepping it around (or you run out of room). I've only done it once, as I have an unhealthy fear of losing all my film in the post (I know it's not rational). Lots of people do it.
5. I have a couple of photos in my gallery from Vietnam, but I'll have to do a search and see if I can find the original scans to post here.