Vac-u-lock is a system for attaching dildos to harnesses. It requires a vac-u-lock system harness and vac-u-lock system compatible dildos. Personally, i think O-ring harnesses are much more convenient and have a wider range of available dildos and harnesses. i've never been interested in vac-u-lock.
I have to second charletnarouh's answer. I've owned a Vac-u-lock and it really isn't great. The harnesses are mediocre, at best, and you are locked (no pun intended) into purchasing their proprietary dildos in order to use them. Often these toys are made from less than desirable materials like jelly.
I personally recommend the Spareparts Joque, or my favorite, the RodeoH harness. If those are too pricey (they're well worth every penny, though!) then perhaps look at one of the Tantus starter kits. The harnesses they use are decent, and fit a large range of sizes, both people and toys.
With an O-Ring harness you are able to purchase a larger number of toys, in various materials. You can get almost anything you can imagine made in silicone that is harness compatible, and you can switch out the rings to accommodate very large and very small toys. You can even get ceramic dildos for harnesses. It opens up a world of options and I think you'd be much happier in the end!
Doc Johnson has been expanding into real silicone toys recently, but I also recommend going with an O-ring harness. Many are well-built (some aren't), and you have a wide variety of bodysafe dildos to choose from.