It appears "vJoyInterface.dll" isn't 64-bit compatible.
This is the log message I'm seeing on my machine:
Plugin initialization exception [VoiceAttack Joystick Output Plugin]: An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)
You could try the 64bit DLL, which is included with the vJoy SDK.
Download the "Feeder SDK" from
here, then copy/extract "vJoyInterface.dll" from "SDK\lib\amd64\" (inside the .zip file) to the joystick output plugin directory in your Apps folder, and overwrite the current file; then check whether the plugin functions (it's not showing the error in the log on my machine after replacing the dll, but I don't have the driver installed).