Camera does not recognize the older flash. The flash pops up because you are in the Auto mode, and Auto thinks flash is needed.
If you use camera modes A, S, P, or M, the internal flash will not pop up automatically. In those modes, if you want it, then you open it, and if closed, it will not bother you.
The Vivitar likely has a very high sync voltage (applied to the camera hot shoe). The Nikon DSLR is rated for 250 volts, probably no damage, but this Vivitar might be rather close to that (modern flash units for digital cameras are more like 4 to 6 volts).
And this old flash cannot do iTTL on the modern camera, i.e, the camera could do vastly more than that old flash can do.
See
Review of the Yongnuo YN565EX Speedlight , in the $100 class, that will run circles around the old flash.
