I kept my white Mandevilla inside last winter, brought the whole pot in a cool basement with a little window and kept it from completely drying out. It pulled through and rebounded once it went outside in spring, and did great. Now it is climbing up a six foot bamboo stake and I have to decide how much to cut off, and bring the pot in again.
Update, 11-2020-- Mandevilla rebounded and grew well this summer in a pot. Needed frequent watering.. upon pulling it now after a month drying out I find it really did not grow roots out much or fill the pot, which may explain why drying affected it so quickly. I took a few long thin, green cuttings end of summer and put them in water. One has rooted, and I potted it today. I will take more cuttings, use hormone, and pot them, but am going to let this 3 or 4 year old plant go- it has orange aphids and I don't need to bring any more of those in than I may have already.
Update April 2022 - I can't remember what became of that last posted idea (pandemic brain) but I think I kept the original rootmass after all and let it grow. And I still had a big white Mandevilla last summer - I would not have bought a new one. I saw very little regrowth this winter (just two very skinny vines) so I thought I would toss it all. But on pulling and prying off soil and roots, I find the original plant has a number of thick tubers. I am again letting it regrow if it decides to.

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