The Vivitar 24mm f/2.8 was another entry in the brand's TX-series wide-angle primes, contracted to Tokina and sold roughly between 1976 and 1980 as a moderately fast wide-angle for manual-focus SLR owners buying outside their camera maker's own lens range.
It is a manual-focus wide-angle prime built by Tokina for Vivitar, with a filter thread that varied between 52mm and 58mm across production runs, and offered in the main manual-focus SLR fittings sold in camera stores of the period. Specific element count, weight and minimum focus distance are not confirmed from available sources.
At f/2.8 it was competitively fast for a 24mm lens of its generation, trading some corner sharpness and flare resistance wide open for a genuinely usable maximum aperture in dim interiors and available-light reportage.
Secondhand examples are inexpensive and plentiful. Worth checking: which of the two filter-thread generations is fitted, whether the aperture ring detents still click positively, and the glass for the haze and coating wear common in lenses of this age.