Heating in the vac seal is very fast and convenient. I use the microwave, some prefer water bath which is slower. I'm not worried about the "plastic/microwave" thing, I keep an eye on it. If you let it come to room temp or close (safer with cooked chicken than raw) it can take as little as 30 seconds for one boneless breast, maybe a minute, unless you come straight from the fridge then maybe a couple-three minutes. I heat until my hand doesn't want to hold it and let rest for a bit. If cold, best to heat for a minute and let it rest, then check it and bump it again until its hot.
I do this with rotisserie chicken. Sam's club has huge ones for $5 last time we got them. I break them down and foodsaver the boneless breasts and the hind quarters. I pick the carcass clean as I can and make chicken salad with the bits, or in the colder months make my own broth with the carcasses then soup with the picked bits.
The moisture level begins with the cook. I've learned to brine boneless breasts for grilling and watch them like a hawk when grilling and yank them at 165. If I check and they are 125 and dare walk away to do something else, they are over 165 before I realize it.