I’m making a toddler vest using the V-neck Tunic pattern by Blacker Designs. The directions say to sew up right shoulder seam first, then knit the ribbed neckband. Then the pattern says to sew the left shoulder seam, and knit the ribbed bands around the armholes before sewing up the side seams. It seems easier to sew all the seams first and then knit the ribbed bands, is there a reason I shouldn’t?
You could do it either way. If you want to work flat, seam after. If you want to work in the round, seam first. Knitter’s choice. Sometimes patterns are written to avoid working in the round; some people have an aversion to it. Sometimes they’re just written they way they’re written. Either way they’re not carved in stone. You can be the boss of your own knitting.
To work it in the round you’d need to adjust the stitch count to eliminate the column of stitches for seaming, one stitch at the beginning of the round and one stitch at the end I expect.
You said it’s a toddler vest. There would be an exception I can think of to my “knitter’s choice” comment. If it’s supposed to button at one shoulder then knitting in the round won’t work but what I read didn’t sound like that’s the case here.