I've got a feeling that you need to set the flash mode to "Manual" in the settings menu. At least that's what I gathered from the manual on line. Then you would need to actually shoot in whatever Nikon uses for manual, metered mode.
One caveat. Be careful with the trigger voltage of that Vivitar. Some of the old school flash units will fry your camera's electronics. the Vivitar 285 HV I have works but am not sure about the 2000.
Pentax K10D, K1000, Mamiya ZE2Quartz, DTL1000
The only reason for time is so that everything doesn't happen at once.AThere are many things of which a wise man might wish to be ignorant.