I made the Kinsman pullover which is much the same as the vest. It was one of my first sewing projects but with some help it turned out pretty well. I don't remember how long it took but it would be a one day project if I did it again now.
As for the the performance of the pullover, I've worn it on days down to about 25* with just a t-shirt underneath and been warm enough. When it gets colder than that I need to throw on a base layer to keep from getting too chilled.
I think if you've completed a quilt you should be prepared enough to make the vest. All of the kits from thru-hiker that I've made have been great, coming with patterns, instructions, and you can get a hold of AYCE very easily if you run into any problems. I'd say go for it.