I use a vacuum for clean up, and used to use it for stuffing. The reality is, after about 2 grams of down enter your vacuum tube, it's basically clogged, and very little suction ends up at the nozzle. So it's pretty much just loud and useless beyond that, you end up either putting 2gs at a time in a chamber, or you continue stuffing the vacuum tube by hand.
Now, I just get down in there, with a tube of known weight, and stuff as much down as needed into the tube. Then I stick a rod of a bit smaller internal diameter into the tube to eject the down into the baffle chamber. I've gotten very good at this, 9 out of 10 times I'm accurate to within half a gram, I can stuff a quilt in about half an hour.
I know there are more sophisticated ways to do this, but any of them that rely on large bags of down on a scale being tared down constantly, is asking for inaccuracy. Also, most scales are very subject to moving off accuracy over time with constant weight on them, and highly sensitive to vibration and EMI. That's why I haven't bothered with a down ejuctor or whatever they're called. I calculate my down to the 1/10th of a gram, on a baffle by baffle basis. I'm sure a very expensive scale could overcome some of these issues, but I don't see the return on investment at my level of operation. I've also never had a quilt end up over projected weight, ever, yet always meeting or beating loft expectations.
For moving from one bag to a new project, if you can get your hands down into the chambers of the existing bag, I recommend just putting it straight into a tube from there, weighing the tube, and adding the down directly to the new project. The trick is to gather clumps with your fingers, while inside the old bag, and stuff them as such into the tube. Big clumps, just force it in there. If it starts getting tight, switch to the other side. Turn your scale off and on between weighing, unless you've got two different scales to verify. They tend to drift off accuracy as mentioned. Once you know the weight of the empty tube, you can use that as a baseline. Re-weigh it every so often to make sure the scale is staying calibrated. Some scales are awesome, some suck. Scales that are only accurate to the gram can give you some big swings, and bear in mind, they won't normally show the next gram until you're .60g over the shown weight.
Also use a tube that's as light as possible but rigid enough to not bend, preferably something that isn't so round it rolls off your scale.
Even working out of 10-20lb giant bags of 900fp down, I never loose more than a couple of grams of down per stuffing session. Yeah, there's a bit of it floating around, that's where the shop vac comes in. Do yourself a favor and just trash this flotsam, it's too dirty to use at that point.