(I swear, I try not to make goofy "before" faces, but it's more difficult than you'd think!)
I started by performing your standard pants-to-skirt conversion. I cut open the inseam, all the way up one leg and down the other. Then, with the pants inside out, sewed straight down the front below the zipper to remove the excess fabric of the former crotch. When I did the same to the center back, I also took out a few extra inches since the waistband was too large as well. This made the back pockets a little too close together for my liking, but belt loop and pocket placement on the hips made the possibility of taking them in on the sides seem like more work than it was worth. I did, however, use my seam ripper to pick out the center back belt loop from the discarded piece and reattach it at the new center back. This helped cover the seam, and of course, keeps my belt in place. Also, when I sewed down the center back, I stopped about where my slit/pleat was going to start. I chopped off about half the leg length for a knee-length skirt, and used the excess fabric cut out of the back crotch to form a pleat to fill in my slit for modesty's sake. Finally, I hemmed up the bottom edge, and my skirt was complete.
Now, what to do with the bottom half of the legs. Surely that's enough fabric for a vest, I thought. I pulled another vest from my closet for comparison, and behold, it was enough fabric! Huzzah! Unfortunately, I can't really explain how I did this very well since it was quite the seat-of-the-pants refashion ("seat-of-the-pants" - ha! puns are awesome!). Basically, I sewed the two pant legs together at what would become the center back of my new vest. Then I ripped open the side seams from the top to about half-way down, and sewed a couple little shoulder seams. I tried it on at this point to make sure it was going to fit. With that confirmed, I cut out curved armholes from the ripped-open side seams, and trimmed out a deep-v neckline. Toss in some bust darts and hem all raw edges, and you've got yourself a snazzy pin-stripe vest. I thought about adding some kind of closure to the front, but I kinda liked the way it hang loosely. Plus, button holes are hard, yo.
I felt rather professional today in my power suit and black pumps. And hopefully, no one realized I had pant legs draped around my shoulders.