I have brought mine in and overwintered in a sunny window.

I have tried without success. I think that the part of my house it was in was too dry. However, the administrative assistant at the school where I previously worked always brought hers to the school for overwintering. It sat in the broad front hall, which was always cool, under a skylight that gave it good light. (She also overwintered bananas, which developed fruit which was fun for the kids to watch, as well as orange trees. She has quite the green thumb and it seemed the perfect spot, cool without getting too cold, a tile floor so no worries about spills and extra humidity from all the moisture that came in on folks' boots, and bright light, or at least bright as NH gets in midwinter.)

I have not bought a Mandevillea in a long time, so have no experience in over wintering it. I did want to mention though that the plant that I have the most success with overwintering, is my tropical Hibiscus. They sit in a sunny West window all winter and don't seem to be bothered by the reduced sun, or the dry air. I pay very little attention to them and just let them get a bit of rest over the winter and when they go outside in spring, they perk right up and get back to performing. And if I've been good to them over the summer, when I bring them in the house, they usually will keep pushing out flower buds and bloom in the house for a month or more. I have an open flower on it right now and another bud still to open.

I have an unheated sunroom that does well for a number of plants. My Meyer lemon tree is at least 15 years old, and right now has 10 large fruits just about ready to pick. My tropical Hibiscus is still blooming and my Gardenia "Diamond's Fragrant Delight" is blooming now as well. So maybe next year I'll try the Mandevillea.

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Last winter I grew some cuttings of a Mandevilla from the summer. Took the cuttings in fall, stuck them in water, and then potted them up. Unfortunately, I damaged most of them, mostly from waiting too long to pot up. Grew the surviving one on a sunny window until Jan. and then under light for the rest of the year. It was fairly stagnant in growth until the spring, and when I put it outside come June, it grew really well and produced a lot of flowers. I'm not doing it this year only because I found I am a bit allergic to the sap. My friend overwintered hers', but it took until late July to really flower well. Just kept in a sunny room.

I saved one from the horse stable I work at! In november it was sad, so I cut it way down and added soil since pot had been dumped a bit. It is in my bay window and growing crazy! not sure if I'll get blooms but at least its alive. Ill keep everyone posted! I was told to give it plenty of sun, but little water? seems to be happy, so far so good! I've also known some folks who let them go dormant for winter with much success. However Im a fairly new gardener and have yet to try. If my plan isnt successful I may give it a shot next year!