Out of Control Vines

Just cut it back to get rid of the extra length - give it a haircut. Probably it will sprout new vines from lower on the existing one (they'll fork off to the side) but at least you can contain them on the existing trellis. If that gets too thick and unmanageable, just cut some of the side shoots before they get too long.

If it's in a window box, what happens if you don't prop up the vines but you let them dangle toward the ground? Vinca plants are vining and meant to hang down, not trellis up, for example.

You can move the window box if you want to, putting it on a railing or a deck, and let it wrap around pretty much anything you want. There are lightweight trellises that are cheap and can be stuck into the ground or standard flower pot with the two bottom prongs. You can also use a tomato cage (the round thing people put around tomato plants to hold the plant and fruit off the ground. You can find old chairs and birdbaths for practically nothing at a yard sale, and lots of people use those for garden climbers.

I'd also consider whether you have a beautiful and sustainable plant, or whether your seeds just never grew and you've got a weed like catbriar growing there! You might take a cutting to your local garden center and say, "Hey, any idea what this is?" Go to a local place, not a big Home Depot or Lowe's garden center where they have a big supply but not necessarily anyone experienced in horticulture. I do that all the time - if I have anything that looks like it has a problem (disease) then I put it in a sealed plastic bag so it doesn't infect anything at the garden center. They appreciate it and are very helpful to me in finding the right treatment.

Just trim it regularly to keep it the length you want!

If you like the vines (the color, leaf shape), sure, replant them to a better spot, trellis and train them. Some plants need to vine out a bit before they flower. If you aren't wild about them, then just trim them back to a size which works for you. Diane B's suggestion of having it identified at a nursery is a good idea. (I don't recommend big box home stores though, ours are lousy and they often sell products containing neonicontonoids, which will contribute to bee die off). A nursery usually has a master gardener on staff. They'll help you make a plan for what you want to do going forward, including any soil amendments or care requirements they might need.

I'd prune the vines to a reasonable manageable size.

Just prune them. You control the growth.