That's a gorgeous vest! Here's its Ravelry page
, in case you haven't seen it.
I have a hard time with decreasing lots of stitches across a row as well. I think there's a formula for this, but I'm terrible at math so I always make sure I have a lifeline
in place and then I just guess how many sts I need in between those decreases. Sometimes it works for me and sometimes it doesn't.