I made a sleeveless shell two years ago. The yarn was pretty, my gauge was correct and the fit was almost perfect-- except for the armholes. The armholes are too big for me.
The neck is a regular round neck and high, so it doesn't look good with a t-shirt underneath it. It doesn't look like a vest at all.
Not sure how to explain this, but the problem is not at the bottom of the armhole but the problem is the sides of the armhole gap open too much. The schematic did not indicate how deep the armholes were, so I didn't know there was a problem until the shell was finished.
My knitting friends think that adding an edging would only add weight to the armhole and make it gap open even more, so I'm going to leave it be.
For future reference, do you know of any instructions in a book or online on how to adjust the armholes on a sleeveless garment?
~Knit for Charity~
"that their hearts may be encouraged, being knit together in love" - Col. 2:2
You could do a simple single crochet edging decreasing as you go in the problem areas. Or you could resew the shoulder seams taking in extra at the armhole edge to compensate and take up some of the gapiness.