1. The Vivitar (available under several names as OEM) is noticeably cheaper.
2. There is no quality control. The Vivitar, I think, can't focus to infinity (only a thinner adapter helps). The Mitakon has a tilted focal plane.
3. The Mitakon is more comfortable to operate, the focus ring and the aperture ring are more convenient. The Vivitar's very strange aperture scale is uncomfortable.
4. The Mitakon has a built-in hood. The Vivitar has a detachable hood (comes with the lens).
5. Both lenses have a 55mm filter thread. But the Vivitar only comes with a 72mm cap for the hood. You should find a 55mm cap if you don't want to use the hood.
6. The Mitakon is slightly larger.
7. The Mitakon weighs 365 grams (without caps), the Vivitar weighs 292 grams (without the hood and caps).
8. The Vivitar has only 6 aperture blades, while the Mitakon has 10.
9. The angle of view of the lenses is virtually the same.
10. Resolution in the center is about the same, but on the edges and especially in the corners the Vivitar is noticeably better.
11. The vignetting of the Mitakon is stronger than that of the Vivitar.
12. The Vivitar's distortion is almost unnoticeable. The Mitakon has a little barrel distortion.
13. Both lenses have a rather long minimum focusing distance (80-90cm). This results in poor macro capabilities (magnification about 1:8, x0.125). But the Vivitar has a little more magnification.
14. Both lenses have noticeable longitudinal chromatic aberrations (see
Vivitar's example).
15. The Mitakon produces better sunstars than the Vivitar.
16. At the same shutter speed and ISO, the Vivitar at f/1.8 is about 0.3EV brighter (in RAW converter) than the Mitakon at f/2.
17. With the same white balance settings the Mitakon has a greenish cast or the Vivitar has a purple cast.
Everything looks like the Mitakon is more comfortable, but the image quality is better with the Vivitar. Still, I want to compare the out-of-focus area blur in more detail. This could be a meaningful difference.