I'm not sure about the reguirements for Vinca but your idea should work if the plant likes shade and is protected. But you should line it with something before you add soil or else you'll get soil all over the floor :)

For the 'trunk' you could use dead branches, dowel rods (sp?), broom sticks ( I guess that a dowel rod, huh?) or anything else that could support it.

If you have the time/money or interest check out The Complete Topiary Book. It has a lot of projects from small Topiaries that you bring indoors and tutorials on making hedges into shapes.

This may be easier than the method you want to undertake:

You need a hanging basket (10 inch), a thick stick like a broom with a flat top, a good sized pot (about 12 inches) and some cement.

Paint or decorate your stick by wrapping it in raffia or maybe twining some dead grape twigs. Anything just so that it doesn't look like a plain boring stick.

Place something inside your pot to cover your drainage hole. Mix your cement and pour it inside your pot, now take the stick and set it inside the cement before it sets. When it sets the cement should prop your stick up.

Now take a square piece of scrap wood (6 inches should do it) and nail or screw it to the end of your stick. Now set your hanging basket ontop of the square piece of wood and from the top of the hanging basket nail or screw it in place.

Remove the hanger from the hanging basket and fill with plants of your choosing or buy a hanging plant to go inside. If you choose to plant it yourself you can choose various flowering annuals to give you some color and interest. But you should have something inside that trails over and makes the ball shape, like Ivy, Vinca or Sweet Potato Vine or Wandering Jew. If you choose the right plants they should grow out and cover the pot in no time. Trim to keep the compact bushy growth.

When the growing season is over you can toss the plants and store your topiary frame until the next season. If you don't want to use cement you can use gravel or bricks. Anything as long as it keeps the stick and pot on top from tipping over.

To give you an idea of what it should look like google 'standard topiary'. You get the same result results but your trunk and ball are "faked".

HTH