Measuring gauge on ribbing is tricky, you don't know how much to stretch it. If the whole thing is ribbed, measure it slightly stretched, as it would be when worn. Also, make sure you're using about 25 stitches instead of just 19 and don't include the edge stitches when measuring. It also helps to wash and dry it like you will the finished vest, that way you can pin it to dry with the ribs slightly stretched and will make measuring it easier.
sue- knitting heretic