I once had an Austin Healy Sprite. You reached more or less down to reach
the know from the driver's position. The handle was mushroom shaped and fit
perfectly into your palm. I later had a triumph with a manly shift lever
that you reached up to to engage. It had a knurled brass (not stock) one
inch or so diameter "knob" that was about three and a half inches tall. It
fell perfectly into my hands grabbing it like a subway pole. Very vertical.

I had a Renault Dauphine with a little teardrop-shaped white porcelain knob
that made one stick one's little finger out to operate the gears like a
demitasse cup.

I had an MGA with a stitched leather ergonomic kidney bean with the car's
logo in the top.

Each was perfect for its task whether it was original or a replacement.

What is the perfect shape for a vanagon, especially something that could be
turned on a lathe but not limited to that?

It could be shaped like the oarlock on a Venician gondola for all I care.
It's too cold and snowy to get out and take measurements, just in the
thinking stage. Think about how you reach down to engage the Vanagon shift
knob (unless you are my wife the time she literally fell off the seat and
onto the floor in traffic reaching for something).

Jim