It's a fairly large switch (not the stereotypical slide switch),
and it's a two-pole triple-throw.

You're sure it isn't DPDT with center OFF (ON-OFF-ON?)

Nope. There are four pairs of contacts (left/right), and (as far as I can
tell) a single slider that bridges two on a side at a time. The switch thus
has three distinct positions. The purpose is to provide "normal" and
"high-power" positions by changing the number of cells. *

After thinking some more, I realized the simplest solution is to drill a
hole and insert a heavy-duty double-pole toggle switch that will bridge the
switch contacts for high power.

* This is, of course, bad design. If you use the vacuum mostly in normal
mode, those cells will wear out much more quickly. Which I why I always run
the unit in high.