Hooking up additional outlets & lights to a central vac circuit?
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!