If you have not replaced the old battery with a new one, I would recommend getting a new maintenence free battery. The VN750 demands a good voltage source.
If you can measure 14V+ with a DMM at the battery when reving the engine to 4K rpm, you likely do not have a charging issue. That said, make sure all connections from the stators three bullet connectors to the R/R, and then from the R/R to the battery are clean and making good contact.
Yuasa YTX14AHL-BS, YB14L-A2, YB14L-B2
GNB 14L-A2, 14L-B2
Interstate YTX14AHL-BS, YB14L-A2, YB14L-B2
GS-Ztong Yee CB14L-A2, CB14L-B2
GS-JSB GM14Z-3A, GM14Z-3B
Motocross M62H4L, M3214Y, M2214B
Napa 740-1851, 740-1828
Sears 44005, 44364, 44059
Yacht CB14L-A2, CB14L-B2
I would also recommend iridium plugs (Part#: NGK DPR7EIX-9), and the ignition pickup coil modification to make starting easier. You can find info on this mod by seaching the site for "pickup coil mod".
Check Kanucks thread for a downloadable service manual:
welcome to the group