Oh, the Inhumanity!
Where I am now: 2 rows and a bind off shy of finishing the vest.
How much yarn is left: 3 inches
What I'm doing about it: If I need to, I will ask Fleece Artist to do another skein for me, but since I'm only 9-12 feet short of finishing (and that estimate is probably excessive), I'm going to wander around Ravelry and email people who have done this vest in the Mermaid colorway and beg for yarn, starting with those located closest to me geographically first. I'm waiting for a reply from a knitter in New York right now.
The moral of the story: If you want to make your Celtic Vest longer, only add 8 stitches to the cast on for extra length.
ETA: I just made a deal for a few more yards of this yarn in exchange for patterns! Yippee, I can finish my vest soon!