I vacuum pack my emergency supplies such as cotton fire tender and gauze to minimize their size. I did a little test and vaccum packed my bivvy and a few other items just to see how much smaller they would be. The results were impressive, but once an item is opened in the field the space savings are lost unless you happened to have packed a vacuum pump. Have any of you guys investigated the trade-off between the space savings of vacuum packing your gear vs the extra weight and space of packing a vacuum pump?

If you're concerned about volume, a nice lightweight compression sack will do the same job
your vac-sealer will do for bulky items.

I care mostly about simplicity and dependability in my gear. Volume is not very high on my priority scale.

I can see vacuum packing emergency supplies such as extra food that may never get used-sealing them up preserves them and keeps them dry for one thing, but as far as anything else, as already mentioned, a stuff sack is better in many ways- it does about the same thing and is much more versatile than a plastic bag.

There are space saver bags that are the best of both worlds. You put your stuff in them, then push the air out of the one way valve. You can reuse them and push out the air while in the woods, away from home. I have some by Coleman that I bought at Wally-world. There are others of course.