Glad you figured out what's going on.
Sorry about the frogging.
But you sure did prove once and for all how important gauge is.
It's late and I'm doing the math in my head so I may be way off,
If you're 1 stitch per inch off-gauge
and the vest is (let's just say) 40 inches around,
that's 40 stitches off.
Divide 5 stitches per inch into 40 and...
your vest would be 8" too big.
But hey -- alpaca is such beautiful stuff; think of it as getting more time to knit with it.
Best wishes to your mother.