15amp switch load clarification
Using 15A light switches to switch &lt;15A on a 20A branch circuit is OK, unless you're switching receptacles!
As it turns out, NEC 404.14(A) permits the use of a snap switch (general light switch) to control any hardwired, non-motor load up to its rating, or motor loads under specific conditions (informational notes omitted):

404.14 Rating and Use of Switches. Switches shall be used
within their ratings and as indicated in 404.14(A) through (F).


(A) Alternating-Current General-Use Snap Switch. A form of
general-use snap switch suitable only for use on ac circuits for
controlling the following:


(1) Resistive and inductive loads not exceeding the ampere
rating of the switch at the voltage applied


(2) Tungsten-filament lamp loads not exceeding the ampere
rating of the switch at 120 volts


(3) Motor loads not exceeding 80 percent of the ampere
rating of the switch at its rated voltage

Note, though, this doesn't hold true when you're switching receptacles, as someone could plug say, a space heater into the receptacle(s) and exceed the rating of the switch as a result (informational note omitted again):

(F) Cord- and Plug-Connected Loads. Where a snap switch or
control device is used to control cord- and plug-connected
equipment on a general-purpose branch circuit, each snap
switch or control device controlling receptacle outlets or cord
connectors that are supplied by permanently connected cord
pendants shall be rated at not less than the rating of the maximum permitted ampere rating or setting of the overcurrent
device protecting the receptacles or cord connectors, as provided in 210.21(B).


Exception: Where a snap switch or control device is used to control not
more than one receptacle on a branch circuit, the switch or control
device shall be permitted to be rated at not less than the rating of the
receptacle.

The reason why is in your hypothetical case, the wall switches are not in the path of the current the vacuum draws, basically.  (Keep in mind that current flows in loops and that loads connected are in parallel.)