I'd plant one vine on each corner, prune the cordons to 3', and let the glory begin!

How crafty are you??? With duck tape and a staple gun you can do anything you set your mind to. Okay seriously, I have found that big box stores (as they are referred to on here) do not offer adequate goods for us plant lovers so, having said that, I would try a nursery and see what they got. However, it has been my experience that they only have small plastic trellis' which are fine for philos and such but not for a 10 footer. So I think your going to have to make your own. Same principle I think would apply to clematis if your are familiar with that. As young plants, they need support to get going so I use chicken wire (because the young tendrils can easily afix themselves to the smaller openings), wood dowels and a staple gun. Chicken wire can easily be cut by our dainty little hands and can be shaped into any shape you like. Another suggestion is to use wooden dowels by themselves and make your own trellis with twine and thin dowels (easy to transport and lightweight if you use the thin ones.) They should be able to support the weight of hoya. Or maybe a moss stick and you could wrap the hoya around that? Whatever you use, you always have the option of putting pot on a base with wheels and that would make anything you use easy to move.

I've got wild grape growing up into some of my conifers in my woods. Can't be allowed to happen as they will grow so profusely as to shade out the underlying tree. So yes, cut and apply herbicide. If you are able to acquire the herbicide Garlon, that is a little more effective for this application than is Roundup. But......Roundup-style herbicides will do the trick. Are you making a strong solution? In the case of professional strength glyphosate (Roundup active ingredient) materials that I've used, the recommendation for this application is to mix the full-strength concentrate 50/50 with water. BTW, I think this problem is on the increase. My theory is that there have been some drought years, and that nothing, not even severe drought stops these vines. Hence, the poor growing conditions of recent years have given the wild grape a bit of an advantage. Not sure, just speculating. +oM

Normally muscadines are at the post with the tips of the vines touching or bypassing each other at the midpoint between the posts. On your question, I'd prune to fit the space. 10 feet each direction is normal for a muscadine. I don't know anything about the other grape.