"If it is installed so the boost goes into the side of the 710N, what is there to keep boost under control when not under vacuum? "

you missunderstand the purpose of a DV. A DV is not for controlling boost really it is only to releive excess boost under closed throtle...
When you are under boost with WOT the pressure in the line is relativly equal to the pressure in the manifold but when the throttle closses. it blocks air intake and the engine creates vacuum in the manifold. now there is still pressure in your lines the DV is only to release pressure under this circumstance. The vacuum from the manifold opens the valve on the DVand releases the excess pressure in the lines.
The wastegate operated by the N75 solenoid controls boost levels by altering the amount of pressure sent to the wastegate actuator.

"leak where? the whole system is pressureized where would the boost go? its a recirculatory valve, if it leaks it puts it into the system, thus losing no pressure. if it was bov then it would leak to the atmosphere. "

Like PaulA4 pointed out it leaks it into the unpresurized pipe before the turbo and after the maf.

 

Gotcha

Thats why i run a BOV. Very easy to understand.