I climbed Fansipan during Tet holiday. From start to finish, I only encountered 2 people, a vietnamese girl and her guide. But now there should be some tourists around and you should be able to coerce someone into joining you. Most of the trekking companies strongly advise against going alone and they recommend 2 or 3 days. If you can follow a trail and keep a good pace, it can easily be done in a day, but some parts have tricky navigation, especially in the rain and whiteout visibility. I hope you have clear skies, the surrounding hillsides are supposed to be breathtaking.