Joe, yes, I do the boilin' bag thing for butts and briskets the same way. I'll vac a whole pork butt minus the bone, press down on it a bit in the bag to separate some of the large muscle chunks and thin the total mass a little, and chill it quickly before I seal it so the juices solidify some. That helps keep juices in the bag and not in your machine. I just went through this process a couple of days ago for my grandmother and some of her buds who decided that they were going to need a weekend pulled-pork fix, in fact. I let a frozen vacced butt sit in the refrigerator for a couple of days to thaw, then simmered it slowly in the bag for about 45 minutes while I was getting dressed. Wrapped the bagged pork in a towel, set down in a foil half-pan, wrapped a few layers of foil around that, put the whole bundle in an insulated casserole carrier, and it was still steaming hot when they decided to open it and eat about three hours later, so she tells me. I don't like to pull it before I vac it, but I do like to at least partially separate some of the larger muscle chunks to facilitate thawing/heating.
For smaller bags of leftover pulled pork, tjkoko is right - 15 minutes will probably do ya. You'll be able to tell by the movement of the juices in the bag.
Use caution when doing the boilin' bag thing on whole briskets, though. I find that briskets work much better if you'll go ahead and separate the flat from the point before bagging them and either lay them next to each other in the bag, or bag them separately.
Keri C, still smokin' on Tulsa Time
Hot Wire BBQ