Yes, taking one, or a couple spaced out, short breaks down south during the height of a cold dark winter can make a big difference, physically and mentally. Perhaps consider a "winter home" since you probably have sun in summers.

One place to suggest for Vita D travel ... if you want something cost effective, hunt around for good airfare and stay in a health resort or use vrbo.com or airbnb.com to stay in the Puna area of the big island of Hawaii. This is a very affordable, downright cheap, area of the whole islands to stay. American's think of Hawaii of being just off the coast of California. The fact is that it is closer in latitude to Mexico City. Great, affordable place for Winter Vitamin D. Full of Minnesotans in winter.

As for PN improvement, at the time I had a couple of emotional meltdowns because I had some instances where I could see that my PN was much worse than I previous thought it was. However, later this year I have had some wonderful PN surprises, where my partner has looked at me and said "look you are (standing, walking, being in the cold, etc) much longer than a year ago ... you could never had done this a year ago". I have been considering writing a post titled "here's some evidence of hope for fellow chemo pn-ers", however, I don't want to jump the gun. I need to properly push myself and test out exactly what my limits are and if/how they have improved.