Though I haven't done a comparison, I believe the Vivitar 400mm f/5.6 lenses are more highly regarded than the f/6.3.
I think they both have a better reputation than the cheap 500mm f/8 lenses.
I have a TX mount Vivitar made my Tokina. Mine is a 77mm filter size.
It produces acceptable results, but is very heavy. I haven't figured out how to stop it down. It has a small spring loaded wheel that you have to hold to stop it down. Very awkward (maybe somebody can figure it out for me).
I think the DA 55-300 might produce better results, if you don't need the extra reach. I don't think there is a big difference between 300mm and 400mm.
Here is a brief test I did when I first received the lens.