I had the same kind of problem with my first 2 sunbursts.. They grew SO freakin' slow.. Brought 'em in for the winter & put 'em under lights.. but one slowly declined and eventually died.. the other hobbled along.. I stuck it outside the next summer & it *finally* took off.

So.. I guess I have no advice, just 'have been there'.

My vines hate being inside over the winter and always look close to death. The only one that held on to any buds once inside was a Lavender Lady and that could live through anything. lol That one is going to get planted in the garden this week and left outside. I'm just going to cut the ones that aren't hardy way back before taking them into the greenhouse and hope for the best. This will be the first year for a number of vines so I don't know what to expect. I'm also going to try rooting some of the cuttings on heat mats with lights.
Karyn