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Karen is correct in most of her answer.It will need support until the trunk forms well.When it is a few feet higher than you want the finished tree to be, cut the top back to the height you want it this will cause more side shoots below the cut.It is at this point that you want to remove any lower shoots that have formed on the portion you want to be bare trunk,up until this point those shoots help to fatten the main vine into a trunk form.The upper shoots that will form the canopy will grow downward in a weeping form as they get longer and heavier.When these hang to the desired level cut the tips.These shoots will then put out side shoots thickening your canopy, and all the energy from all the new foliage will quickly thicken the single trunk.As each set of new shoots begins to hang to low,cut them back and keep any shoots that bud out on the trunk pinched off.As Karen said it is a never ending process.The top however is not supported it is allowed to form a weeping tree style.

Give it something to climb up, something fairly sturdy such as a fence post, and let it climb up that for a bit. Trim off all but one ‘trunk’ after it’s plain which is the biggest one, and also all side shoots from the ‘trunk’. That way it will put all its energy into that one trunk. After a while that one trunk will get bigger and may be able to support itself. You’ll have to keep trimming off side shoots forever, just allowing leaves and blossom at the top. It might help to give the top parts something to climb across; that way you can sort of shape it to be whatever shape you like, and the top parts won’t grow all the way down to the ground again.

You could try a Jacaranda tree. The flowers look very similar to Wisteria.