The Vivitar 28mm f/2.5 was part of the brand's TX-series wide-angle primes from the late 1970s, made under contract by Tokina and sold through independent photo retailers as a faster alternative to the more common f/2.8 wide-angles of the day.
It is a manual-focus wide-angle prime with a 58mm filter thread, built by Tokina for Vivitar and produced across the principal manual-focus SLR fittings sold between roughly 1976 and 1980. Specific element count, weight and minimum focus distance are not confirmed from available sources.
The f/2.5 maximum aperture is a stop faster than most third-party 28mm primes of its generation, useful for low light and shallow-depth framing, though wide-open corners show the field curvature and vignetting typical of fast wide-angle designs of the period.
It remains an affordable manual wide-angle on the secondhand market. Worth checking: aperture ring click detents for wear, front element coating for cleaning marks, and confirmation of which mount the sample carries, since Tokina-built Vivitar primes were sold across several systems.