The Vivitar isn't constant aperture, so it is only f4 at the long end. It is also known to have some pretty strong color fringing on digital due to the coatings, which wasn't a problem on film. The Tamron and Sigma have better coatings to prevent this fringing. You're comparing brand new $700+ lenses to a 30 year old lens that sells for a little over $100, you aren't paying $600 just to get auto focus. The Vivitar was an awesome zoom in the days when telephoto zooms weren't very good, but I wouldn't expect it to compare well to a modern high end zoom, it sure beats the cheap ones though.
I love my Vivitar, it is very sharp, it has 1:2.5 macro capabilities and it cost me virtually nothing.
The Sigma and Tamron are most certainly better, but if they are $600 better is up to you.
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