1. the same design and specifications
2. both with a Pentax K mount
3. the scale marks on the barrels
4. the two lenses, in my eyes, were not much different in the images shot indoor at f1.7, 6 and 22.
1. brand names on the ring around the front glass
2. the rubber surface patterns on the focus rings
3. the coated colors on the front glass: more blue/green on the Vivitar vs. more brown/pink on the Promaster
4. more brown color on the rear glass of Promaster
I also compared three f1.7 k-mount lenses from Pentax, Promaster and Vivitar in my another post.