Clean the vents with an enzyme based cleaner such as Odor Mute, Simple Solution, or Nature's Miracle etc.
This next info might work for you to out think the cat.
Might mean the hall will be a bit chilly while you work on this.
I would suggest covering one of the heat vents (the one furthest from where you can have a box set up) with something to make it too uncomfortable to use upside down plastic rug runner with the pointy sides up might work for example or a baking sheet taped down to the floor.
Over the other vent hole put your box and stick the vent cover in it over the litter. Fill in the litter so that the cat can feel the vent and yet still be using the litter. Alternative is a cake rack to put in the box just along the surface of the litter. You want the ridged metal feel and a cake rack might be cheaper to replace than a vent.
If the heat is critical to your home you can try a cardboard box over the vents with holes punched in the sides instead of covering them. Or you can make a cardboard 'tent' or use one of the plastic covers that points the flow of air in one direction. Fasten cardboard if used to the floor with duct tape so the cat cannot move them.
Once the cat is using the cat box with rack regularly - move it a few inches in the direction you want it to be. Cover the vent opening so its not usable by the cat.
You can usually move a litter box about a foot a week.
You may be able to get rid of the rack after a while.
To help with the transition you can see if using Feliway cat appeasing pheromone will work.
http://placervillevet.com/feliway.htm
Hope this helps you!
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