To control a "universal" motor or a d.c. motor all you need to do
is control the operating voltage.  A universal motor is a motor with
brushes that will run on either a.c. or d.c.  This is the most common
type of motor used in centrifugal fan type vacuum pumps.  Very new or
sophisticated designs may use other motor types but they are likely
to show up only in very expensive industrial units.

There is no reason that a router control would not work as long as
the current rating is sufficient.  Also, a lamp dimmer for incandescent
lamps of suitable capacity would work.

The simplest and the most reliable would be a variable autotransformer.
These are most commonly seen as "Variac" adjustable transformers and
can be found at any electronics or well equipped supply. They also show
up at surplus sellers and on E-Bay.  The advantage of these is that
they have no electronics and are very reliable.  They are available in
protected cases with a plug and socket or as separate units that must
be installed in an enclosure.

Best regards,
Spencer Chase