I will follow this thread because I'm curious as to the answers, but by no means an expert. I brought a smaller (8" pot) mandevilla in the house as well. Zone 6. It's in a sunny window, and I haven't overwatered it, but recently it, too, started to send out vines (I have about 4-5-6). So I'm assuming it's loving the window and going to survive?

I would think that you would want to leave the vines, but I DID chop off the top of the first vine it sent up, is it was several feet and I would like to contain the plant this year to about 3-4 feet. Mine was gorgeous when I brought it in, but forgot to water week one, and all the blooms fell off. it's not blooming, but if it is happy overwinter inside, Im ok with waiting til next year for blooms.

At a school where I formerly worked, someone brought in a Mandevilla, along with other tropical plants, to overwinter in the wide main entrance hall every year. Although there was a double set of doors, there was still a fair amount of cold air that made its way into the hall (a high traffic area) along with a fair amount of snow on boots, etc. The light was from a large overhead skylight. The combination of cooler than normal indoor temperatures with higher than normal indoor humidity, a fair amount of air movement and watering only once a week seemed to keep it quite happy.

So I am trying this out with a large Mandevilla I had in a pot this summer. It is in a SW window in a room that stays between 60 and 65 and within a few feet of a humidifier. I only water when the soil is dry when I plunge my finger in, as previous experience tells me that Mandevillas are intolerant of too much moisture. So far so good after 7 weeks.

I'm overwintering two Mandevilla's. I cut them WAY back when I brought them in. Both seem fine and have new little vine "fingers" reaching out---

My relatively small mandevilla is happy as a clam in my front bay window and is sending up many (10?) "Vines" of new growth.