I figured it out!

The two tweaks that I couldn’t think of yesterday were (i) changing the total width of the panel and (ii) shifting things vertically.

Yesterday I tried to add increases and decreases to shift the 11 + 11 stitches in the two halves into 12 + 10 and back, to make a curve in the middle. Today I took away two stitches from one side, so the curve is between 11 + 9 stitches. That left me with a shorter stem for each leaf. To make the stem actually reach the leaf, I put the leaves vertically closer to each other. In my first attempts, there was only ever one leaf going on at a time. Now the right-hand leaf starts before the left one ends. Not only did this give me that curved vine that I was after, I also got a tighter pattern overall, with less dead space.