Yeah, them nicons are pretty nice cameras.

The serial number indicates the lense was made for Vivitar by
Kino Precision Optical in Japan.  They also sold lenses under
their own brand name of Kiron, and are *extremely* well
regarded.  (Kino/Kiron lenses have a bit of a "cult" status.)

That particular Vivitar lense comes in more than one version.
The Kino f/3.5 version has a macro lock mechanism which requires
the lense to be zoomed to 210mm in order to engage the macro
focus ability.  It began production in 1974.

In 1982 Vivitar switched to a Tokina manufactured f/3.5 version.
It only focuses down to 1:4 magnification, but is smaller and
weights less.

In 1984 Vivitar switched to a Komine made f/2.8-4 lens.  This is
reputed to be better than the other two.  It is large like the
Kino made version, focuses to 1:2.5, and is f/2.8 at 70mm.

The Kiron branded version is f/4.  I don't know much about
Tokina's own branded version.

All of them are fun lenses to use... if you like manual focusing
lenses.  However with a D50 there are two problems.  It is
difficult to manual focus with any Nikon DSLR because they
expect to use AF lenses and do not have a microprism focusing
aid on the viewing screen.  However, it is possible to replace
the viewing screen,

 http://www.katzeyeoptics.com/cat--Nikon-DSLRs--cat_nikon.html

(I have not tried that product, and can only assume it functions
as described.)

Unfortunately the D50 (and D70) Nikon cameras do not couple the
exposure meter to AiS lenses.  That doesn't mean they can't be
used, but using only the histogram for exposure metering is
significantly an inconvenience.

I've never researched that particular lense.  The serial number
indicates it is made by Cosina (which did not have the
reputation that Kino or even Tokina had).