where exactly are you getting power to the rectifier at? A1,A2,A3, or at white wire? if you are getting power at the white wire, its just residual feed back from the battery... There isn't a normal alternator on most motorcycles (the vn7xx included). Instead we have a Stator which produces 3 Phase AC voltage, hence we have a Rectifier which in turn converts the ac voltage to DC voltage, then the Regulator regulates the Voltage to maintain the proper battery voltage dependent upon load of the electrical system.
In theory, the Stator and Charging system as a whole should not effect the engines ability to start. Common problems to week starts is:
Bad Frame Ground
Bad Engine Ground
Loos Battery Terminal Connections
Bad Starter Motor Connections (Ground or Positive)
Bad connections in the Ignition Switch
Bad Contacts inside the start button switch
Another question. How do you know the battery is good? Has it been load tested? Have you tried jump starting it with an car battery?
Also you say its loosing power. How so... you crank a couple of times and the battery is dead? Or you let the bike sit over night and the battery seams week, then you recharge it and its fine?