Several readers have responded that an ordinary incandescent
light dimmer can be used to control the speed of a series wound motor.
These dimmers typically work by cutting off a portion of the AC cycle.
While these dimmers are capable of dimming lights, they may not be up
to handling the higher current of a motor.

Tool supply stores, hardware stores, and perhaps large home centers
carry heavy duty controllers intended to control the speed of routers.
These are relatively inexpensive and are not much bigger than a pack
of cigarettes, and come complete with a power cord and an outlet to
plug in the vacuum supply.  The triac or SCR's inside these units are
attached to adequate heat sinks.  If you want to try a light dimmer,
be aware that the thin front panel is used for the heat sink, and to
reduce the chance of burnout the unit should be attached to a metallic
box to help dissipate the heat generated.

Another choice may be an overhead fan control, although I am not
familiar enough with these to know how they work or even if they would
work in the intended application.  My thought is that the motors in
fans are induction type.  You cannot simply decrease the voltage or
a portion of the AC cycle to reduce the speed in these motors as they
rely on frequency, not voltage.

Tom Rosenbusch