If you can bear the thought of sacrificing a frame or so, do a 'clip test' Clip off a few inches at the lead of the film, and develop it for a typical time for a meduim speed film in the developer of your choice. If it is medium speed, ie. plus x or fp4 film, then the image should look good in terms if density. Check how developed any edge printing is, in case the image exposure was grossly underexposed on the first frame, or you have no image, in the frame, as can be the case if the film was loaded into the camera originallly in a darkroom. If the film was actually a high spped film, then it will need more development time. Look to the data for the developer, and develop for a time similar for that listed for hp5 or tri x.
I have never seen a Vivitar B&W film. I suspect it was a re-packaged film manufactured by somone elese. In thier 80's heyday, Vivitar was a brand that contracted with different companies to make the gear that got thier badge.
