Technique for filling vacuum seal bags without getting any food on the rim

It's hard to answer without seeing what your bags look like or how rigid the material is, but often it's useful to fold the top of the bag outwards over itself. Putting the bag in a suitable container or cup can also help keep things stable whole you fill it. This also makes it easier to wipe up any traces of food that aren't where you want them.

If you search online for methods to fill a piping bag you'll see illustrations of the basic method I'm describing.

I don't have a vacuum system, but when using ziplok type bags I use a wide necked preserve funnel for wetter items like soup or casserole, and improvise a temporary inner sleeve for things like chicken pieces or chops. So if I'm using a larger bag I might curl a silicone baking sheet into a tube and insert that into the bag mouth to hold it open and protect the edges, or for smaller bags I might sacrifice one bag by cutting the ends off to make it open at top and bottom and use it to line the top of the bag I am filling, protecting the surfaces which need to come together.

you can also find 'bag fill funnels' online.

I'm going to re-evaluate the assumptions on this one:

It is my assumption that a good seal requires a perfectly clean rim.

This hasn't been my experience. It certainly helps to keep the rim as clean as possible, but the biggest factor to keeping a decent seal has been the quality of the equipment.

The first vacuum sealer I had was a cheap one to see if the process worked for us. this was alright, but the heating element was fairly thin, which meant that any debris in the area caused issues with the seal.

That failed after about 6 months (mainly due to use). It showed that we used it, and bought one with more of an eye to reviews and value, rather than simple cost.

The new unit has a heating element that's maybe 5mm, rather than around 1, and that seems to give much more leeway. It'll still fail if the rim is too soiled, but a little care, and a wipe down is typically more than enough to keep it in check.

I use a large french fry scoop as a funnel. It's shape slides easily into the bags and, because it's open on one side, it's much easier to remove it without having anything drip onto the bag.

I like the freezing into pucks idea, for those things amenable to doing that.

To re-address the folding over the top of the bag approach, I believe I'm correct in saying that each time mentioned here, folks are saying "fold over the top toward the outside so an inch, say, of the inside now makes a rim facing out" (so a single fold) and the failure noted is that drippage/smearage can just as easily drip or smear on that "inside" (facing out) before even formally starting into the remaining bag (whose inside is still inside...) (where it also drips/smears, leading to the entire question).

Well, yes. But if you fold again, so now (using the inch fold over above) one has two inches at the top folded over with the first inch tucked up in between the second inch and the remainder of the bag.

Then drippage/smearage may very well happen on that inside-facing-out second inch as well as the remaining inside of the bag. But it won't happen (without a lot of help) to the first inch which is folded up to tuck between the second inch and the bag.

Protected like that, one would seldom see a problem just unfolding and sealing, but could though if careless in several ways. So before unfolding, a careful person could wipe the second inch and inside the top area of the bag, then unfold and handle the top inch in a manner that keeps it on the up side of gravity's influence so it remains clean. Nothing within an inch or two of it, should have time to seal before something runs up.

That said, I've also seen the vacuum pulling quickly draw rivulets of fluid toward it which might be the real concern here. But if one is not careless enough to allow that or situations leading to that, the double folding in "fold over, then fold that up under" would in any reasonable circumstance keep the leading edge, where the seal is planned, clean.

I use vacuum bags/sealers all the time. I don't use them for sauces or liquids. So I find even a very wide mouth funnel (like a canning funnel) too awkward to use. A big sloppy, marinated steak is not going to fit.

I'm not overly careful and will sometimes smear the top of the bag. It's sufficient to just take a cloth or a paper towel and wipe the sealing portion of the bag clean after everything is inside. You don't want a pool of liquid or a big mess, but it's not a problem if a touch of moisture remains.

Don't try to prevent it 100%, just clean it if any gets on the bag.

When you fold over the top you don't crease it. It's a lazy fold-over.

I use a 2L drink pitcher for my big bags if I'm sealing something especially messy like marinated meat. Lazy fold it over the pitcher lip, use a rubber band to keep it secured(could also use binder clips), then using tongs I drop the meat in. If there is anything on the bag on the lip or outside of the pitcher I'll wipe it down with a paper towel. If the items to be vacuumed are wet, I'll make sure to use the moist and/or delicate setting on my sealer.

If the food is a bit sloppy and I am afraid that the bag won't seal, I first freeze the food on a cookie sheet or in a muffin tin to make little "pucks". Pop the frozen (or partially frozen) food into the bag and vacuum seal it. Then there is never a problem with grease in the sealed area or liquid getting sucked into the vacuum chamber/pump.

A big sealing machine is also helpful as it can accept wide bags that are easier to load food into without contacting the seal area. I use a huge Cabelas vacuum sealer after burning through numerous cheap units.