Cut off the short neck, or leave it if it's small enough to be covered. (That would make things simpler and add stability!
Find a donor doll with a neck the length you want and cut it off.
Also cut off some extra body plastic, into the tiniest bits you can. (Doesn't have to be much, you'll only need enough to fill gaps or spread over the join.)
Melt the body bits into goop with pure acetone in a glass container with lid. (I like sample-size jam jars, personally.)
Doesn't take much acetone, just a few drops will do it. You want a smooth, putty-like consistency. It's very easy to go too heavy with the acetone, but it will evaporate fast so don't worry if your goop is too liquid on the first try!
Dry-fit the new neck to find out if you'll have to do any filing or not.
If it seems iffy, trim and file things until you get a fit you like! (of you go too far, you can fix with acetone goop later!)
Attach the new neck! If you were able to fit the new neck over the old one, just use a super glue you trust to join the new neck over the old one and let it cure.
If you had to cut off the old neck, you can do this neat trick: Briefly soak the bottom of the new neck in pure acetone and then stick it on the body! Use an acetone-resistant tool to smooth the bottom of the neck to the body. (Tricky maneuver, may want to practice on scrap dolls before attempting on your main custom.)
OR, insert a skewer or chopstick into the body thru the neck hole so it hits the bottom of the inner doll cavity, secure with epoxy putty and trim to correct length that won't interfere with new neck length. Cover top of skewer with epoxy putty and attach new neck over it!
NOW, fill in any gaps and cover seams with your acetone goop!
Let goop harden completely, then sand down any overfill.
By repeating the gooping-sanding process, you can blend away the join pretty seamlessly.
If things get too shiny, matte the whole torso down with a light buffing from a dry melamine eraser.
Sculpting onto vinyl doll faces:
Adhesion is going to be your trickiest opponent here. The squishier the base head, the more of a pain it's gonna be, especially if you're gonna need to paint the sculpted parts to match the skintone.
I've tried a number of clay types, and at the moment my preference is Greenstuff/Kneadatite. It's very sticky after you've first mixed it together and will adhere to most surfaces, but it also holds shape well and is very tough once it cures.
Painting over it can be tricky, though. It can cure shiny and need to be sanded matte, plus there's the issue that it dries dark green. So it needs to be covered with a good neutral primer, preferably one that can be hand painted or airbrushed on--rattlecan primers often have additives that play poorly with vinyl.
Whew, sorry for rambling so long. Hope some of that's helpful! ^^;;