Not sure about your other plants, but the Rangoon Creeper will be very happy with warm temperatures all the time. In fact, I was thinking it might have been responding to lower night temps when you put it outside during the summer and you said it wasn't happy. Your house probably stays a little warmer at night even if you have the windows open.
Near Portland, ah, yes! You probably have a lot of grey, rainy days in the winter. (I'm originally from Vancouver BC so know that weather well
) So your grow light will be necessary even if you could find a brighter window for it. If you want it to bloom, though you might look at putting it under some big fluorescents instead. A ballast from Home Depot with four T6 bulbs in it will come a lot closer to supplying "sunlight" for a big plant like that. Your other plants will love it, too.
If it's been in the same pot for the whole time you've had it, I still think you should put it on some newspaper and slip it out of the pot completely so you can get a look. You've been cutting back the top growth, but the roots could be going around and around in there with very little soil and you wouldn't be able to see it just looking through the drain hole.
