Welcome to Knitting Help!
I'd put the seam on the RS under the collar so that the collar rolls over the seam and covers it (RS of collar facing WS of vest to seam). You could use a back stitch to stitch the collar (seed stitch) to the body of the vest. Pin and baste it in place to make sure you like the way it lies on the vest and then go ahead and seam.
It's a very neat pattern and it'd be great to see a photo when you finish!