Vivitar was a lens designing and marketing company. All their lenses were made for them, by various contract manufacturers. You can easily google for a list of manufacturers by serial codes. Lots of pages of info rating most of the lenses, especially the various Series 1 models.
I use two Vivitar legacy prime lenses, and am very happy with both. But personally, the tele zooms are too big, heavy and slow for me to consider for m43. I feel you would get far better performance with a native zoom, for not a lot more money. I think you can get a Panny 45-200mm for about $200 at the moment, and their 45-175mm for under $300. Lot of lens for the money!