Why doesn't this vector show a fill?

Makes sense  if it's showing fill only where the corners round off, your path probably isn't fully closed. Figma only renders fill inside a closed loop; an open vector path will only "catch" fill in the spots where the outline happens to overlap itself. Select the shape, then in the Path menu try **Close Path** (or select all points and hit Ctrl/Cmd+J) — the fill should apply cleanly across the whole shape after that.