Vivitar made a number of high quality zooms under the Series I label. The 90mm to 180mm f4.5 flat-field VMC lens from 1978 is an one of them.
This Vivitar 90-180mm f/4.5 lens was really designed for medical photography needs. A ring-light flash unit was designed to mount on the front of this lens too. This original medical market niche explains some of the design features and range of this high quality zoom lens.
This lens is long, and pretty heavy. It produces excellent results in macro-work down to 1:4 at 90mm and to 1:2 at its 180mm setting.
Most current zooms only offer macro settings at either one end or the other of their range. Only a few zooms offer continuous macro-settings over their full range. Even the better current zooms generally only get to 1:4 to 1:6 reproduction ratios (.e., the object is one-fourth lifesize on film).
(Source: Cult Classics in Third Party Lenses by Robert Monaghan )
This is a great lens with wonderful sharpness and colours , not reach prime macro lens quality, but very close for that.
I just got back my first roll of slides taken using my 'new' Vivitar 90-180 flat-field and I am amazed... I shot Fuji RDP3 in my OM-4, outdoors, mostly of dead cactus and flowers, a couple portraits, and the sharpness is incredible. Comparing the slides to shots from my favorite lens, a Zuiko 90 macro, I have to say the Vivitar is as sharp, though not quite as punchy color-wise. At f11 the dirt and roots on the uprooted plants is so sharp I can count the grains of sand in the soil.
I have a friend who has two of these lenses, and I've used them several times (on Canon bodies). They're unbelievably sharp for a zoom.