I don't know this particular plant, so if there is something that a creeper owner knows about this, please speak up!!!
That said, I'm pretty sure these are easy and hardy plants, and almost weed like as most vines are. So I'd go with the same advice as most trees, and do it at bud swelling time. If we get some frosts after you have done this, just move it back into the storage and do the two-step until warmer weather arrives in full. (you do know the two-step, right?) A seedling heat mat can help to get the plant established quickly after repotting in our climate, with it's spring swing temps.
I have a Virginia creeper that is pretty hard to kill. I've never transplanted in winter, but I have in spring, summer, and fall to no ill effect. Mines in the ground right now and is small like your but has some serious bark as it older. Just plant it in the ground and let it grow free and hard prune in fall.
A seedling heat mat is different than a higher priced propagation mat. The seedling heat mats, just heat up about 10 -20 degrees above ambient temperature. Propagation mats need a thermostat to control them to your desired temp.
http://www.amazon.com/Hydrofarm-MT10006-19-1-2-Inch-Seedling/dp/B0001WV010#
http://www.johnnyseeds.com/p-9091-redi-heat-heavy-duty-propagation-heat-mats-12-x-5.aspx
I see...thanks Judy. Was considering pulling the seedling pad from under tropicals if temps went wack after I root pruned the creeper and lilac. Wasn't sure if I needed the thermostat then...I should be okay you think?
Judy they are inside...was trying to figure out a way to bump the ambient temps when arctic air moved into our area. Our heater doesn't keep up for the Hawaii bougainvillea...more cold sensitive. Thought to add the heat pad to bump up ambient temps around it...but...no. I doubt it would work that way. Thus I thought I could borrow that pad for the creeper if I need it.Not sure what the question is here. Are your tropicals not inside?