After starting all over with the overlay (**cries a little**), I finally figured it out. I used the same pattern as I did for the underdress, only I cut the pieces off at about 40" (or less) to save fabric. I also cut the front piece down the middle and gave it a 1/2" seam allowance so that I could keep the seams accurate to character.
Once everything was sewn together, I tried it on and marked where the "V" shape would start, about 2" below my chest. I seam ripped the front opening until about 1" below my mark and then put it on my dress form to make the final adjustments. I simply folded the material under, working at an angle from where it splits (the chest area), down to the tail. It's important to make sure that both sides match as closely as possible while doing this, and it wouldn't hurt to get a second opinion about the consistency of those angles. (I used my husband for this.)
After everything was well-pinned, I turned it over and cut some of the bulk fabric, being careful not to cut away too much. Leaving at least one inch for seams and adjustments is a neccessity! I marked where the fold lines were and seam ripped the back and sleeves so that I could lay it all flat on the table to check the angles again. By doing this, I could readjust the pins to make sure the fold lines ran flawlessly, and at a straight angle. Once satisfied, I hemmed the lines up and threw it back on my dress form for a final look. I also pinned on a temporary zipper to hold it all together. This will get replaced by eyelits after I make the V back.
This is what my final pattern looked like. Your own pattern will vary, depending on your own size and height.
And here is a picture of the final mockup. The temp zipper is clumsily laying over the back seam, but it will be replaced with eyelits and the V neck will be cut to match the underdress.