Is the vacuum unit fastened in place?
NEC 2011 210.23 Permissible Loads. ... A branch circuit supplying two or more outlets or receptacles shall supply only the loads specified according to its size as specified in 210.23(A) through (D)...
(A) 15- and 20-Ampere Branch Circuits: ...
(1) Cord-and-Plug-Connected Equipment Not Fastened in Place. The rating of anyone cord-and-plug-connected utilization equipment not fastened in place shall not exceed 80 percent of the branch-circuit ampere rating.
(2) Utilization Equipment Fastened in Place. The total rating of utilization equipment fastened in place, other than luminaires, shall not exceed 50 percent of the branch-circuit ampere rating where lighting units, cord-and-plug-connected utilization equipment not fastened in place, or both, are also supplied.
Utilization equipment is anything that connects to AC power.
You can have more than one installed device on a circuit, but if they total >50% of available ampacity, the circuit must be dedicated to them. In this case, a good installer will install a single (not duplex) receptacle.
If the water heater is already sharing a duplex receptacle with your 12A load, then it must be either
- less than 4 amps, or
- an MWBC. (multi-wire branch circuit, 2 hots sharing a neutral).
Most likely the manufacturer chose 12A because 125% of that is 15A, which makes it the perfect size for a 15A circuit. 125% because continuous load. Not that I vacuum for >3 hours at a time!