I recently saw someone wearing a double breasted vest and immediately knew that I wanted to make one for myself using my Quinn vest pattern. You can check out this reel to see my inspiration and thought process.
It turned out pretty great and was a pretty easy hack in the end. I used a 5 oz cotton linen twill from Blackbirdfabrics for this version, but plan to make another one in some striped linen soon.
OK, lets get started.
Grab all of your pattern pieces. We will only be making adjustments to the middle front and front facing pattern pieces. Starting with the middle front, tape a piece of paper behind it as shown below. Extend the neckline and bottom angled edge until they meet. Adjust this to your liking and trim the pattern piece.
Copy this addition to your front facing.
Next, cut out all of your pattern pieces and start sewing up your Quinn.
When you sew the facing to the middle front it will look like this.
Next, attach the remaining pieces according to the instructions.
Before adding the hem facing, try it on and see if you want to adjust the angle of the bottom overlaps. I ended up taking about 3/4" from the point as shown below. 
Then add the hem facing.
All that is left if buttons. With the right side overlapping the left, place both of your buttons. Starting with the main outside button, mark and sew the buttonhole on the right flap. The button will be sewn at it's coordinated spot on the left flap.
Situate the other button placement. This one will be a faux button on the front. Sew in place through the right flap only.
Directly behind this button, sew another button with a coordinating buttonhole on the left flap. So this button will button to the inside of the vest.
Buttoned up it will look like this.
The last thing, which is optional, is to hand sew a little snap along the neckline to keep the vest from gapping open. I found it helpful.
OK, that is it. Hope this was helpful.