In the photo above you can see the folded sloper patterns taken from the doll, laid out under a finished vest pattern. The back of the vest, in the center of the photo, is designed to fit closely to the doll's torso, so the pattern section here matches the sloper pattern. If you want deeper armholes or a lower neckline than on your fitted sloper, you can draw those onto your vest pattern using the sloper as a guide. You know the sloper is the exact match to the back of the doll as far as the side seams.
The front pattern of this simple vest doesn't need fitted darts. The important fitting line is the side seam and underarm. As I want my doll's vest to hang open like a bolero, I have intentionally rounded the front of the vest from the bottom of the neck down to the waist. If you aren't sure how you want the front of your doll's vest to hang, draw your initial pattern to follow the center line of the front sloper. You can trim the pattern to suit the doll once you test fit the underlining pattern to your doll. As I know the vest will not be tightly fitted to the doll's waist, I have extended the back waistline across the underarm to meet up with the waist line at the base of the front sloper. If you are unsure how this will fit your particular doll. mark the waist line in pencil, but do not trim along the waist line until after you have test fitted your pattern on your doll.
Adding a Western 'Yoke'
My doll's vest will have either a sewn or glued seam at the shoulders, where I do want a neat fit. I have marked the back shoulder seam line from my sloper on the new vest pattern, but I have extended that line past the shoulder seam so that it will overlap the front of the vest and make a wide "V" or arrowhead yoke, common in western style clothing. As I will take the back shoulder line down to create the yoke, I don't need to add a seamline to the top of my front sloper. I can just mark the normal shoulder line from the sloper and glue my extended back seam to that.
Test Fit the New Vest Pattern On Your Doll
Once you have a new pattern drawn out, trim the underarm and necklines carefully (you don't need a seam here as you are using non woven fabric) - when you go to using woven or knitted fabric you will need to 'face' the underarms to keep the fabric from fraying, so you will need an extended seam there. You can leave extra fabric below the waist and past the center front if you want to adapt your vest shape on the doll. (It is a good idea to do final front fittings from the rough pattern on dolls with large breasts!)