Linda - get a grip! You can finish this vest.
1. About the fit - you've got a steek up the front, right? At my gauge the pattern repeat on my Oregon measures 3". How much too big is your vest? You can easily eliminate one (3") or two (6") repeats on the part you've already finished and continue on in the new size. You'll need to take out whole repeat(s) but since it's meant to have some ease, an inch or so either way won't make too much difference. You'll in effect have a really wide steek on the already finished part. So you will mark the new steek edge at the beginning of the pattern repeat that will become the new center edge and add 3 or 4 stitches for the new steek allowance and start knitting up from there. Then when you finish, cut off the extra width on the bottom part. If you need any help (other than knitting it), let me know.
2. Knitting the rest - just think about the remainder as 3 or 4 pairs of socks because that's about what you have left to knit. You knit socks on size 2s don't you? Eat that elephant one bite at a time ;-)
3. Lots of reward. Wouldn't you rather have a beautiful vest that every one can see rather than socks no one sees? I absolutely love my Oregon cardigan and I get lots of compliments on it. The weight is perfect for our weather and I wear it from September thru June.
Just do it.
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