Actually, let me applaud your wish not to have visible stitch lines, but then turn around and say that doing so is not inaccurate. Also, in the end, it doesn't give you as clean an edge as you would think.
Even with an inch hem allowance and cutting notches, the fabric will still pull and pucker. As a professional, even I have a hard time hemming curved edges cleanly, even with rolled hems and notches and every last technique. The way to get REALLY SUPER clean edges on a vest - as well as not have visible stitching? Line it. Make a complete lined vest. It's a little more challenging for a beginner, but if you follow a pattern for a lined vest exactly, it will give you the look you want and be very clean without any notch-puckering.
But in the end, don't be afraid of visible stitch lines on hems. Most artists just don't draw them in because it's somewhat expected of clothing that it has hems. It's also a pain the butt to have to draw such universally-accepted lines into a design over and over again for manga panels or animation. Very little clothing has invisible or blind hems, and a lot of garments you might think has no hem or an invisible hem is actually lined - and that's why you don't see stitches.
For Luffy's vest, which I have done for myself, you can either line it or face it. Facings, for a beginner, might be a little harder than lining because making arm-hole facings and neck facings on a vest require some tricky geometry. But that's how my vest is put together, since I didn't need a lining. Mine has armhole facings, and a front-and-neck facing. Either method gives you really clean edges on curves, and has the added bonus of giving you a reinforced layer at the front edge for putting buttons and buttonholes in safely.
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com
Director, SF&F Masquerade
- COSTUMECON34 - Madison, Wisconsin - May 6-9, 2016 - www.costumecon34.com