I cannot answer generally.
But in the eighties I bought (new) a single coated 4-lens Vivitar TC, and used it quite often for many years.
It offered (on FF):
- Slightly reduced resolution/sharpness
- Quite visibly reduced border/corner resolution/sharpness
- Strongly reduced contrast
- Strongly reduced colour saturation
Later, with APS-C, most of these disadvantages were not such a big problem anymore, because of the smaller format and the possibility of post editing.
But I sold it and bought a used Kiron multi-coated 7-lens TC with KA mount.
The reason for this was not so much the hope for better IQ; the lack of A contacts - making every lens a M-lens - just became a real nuisance for me.
The 7-lens TC offered (compared with the 4-lens Vivitar):
- Slightly improved resolution, specially off the center
- Slightly improved contrast and colour saturation
Now I managed to get (cheaply) the
Pentax Rear Converter-A 2X-S.
This is SMC coated, and (as far as I know) also a 7-lens TC.
It is a real improvement in every respect compared with the Kiron (and even a bit smaller).
And there are worlds between the Vivitar and the Pentax.
Depending on the lens it is used with, resolution degradation can only be seen with serious pixel peeping.
To your question:
If you are speaking about an older Vivitar 7-lens TC, its optics may be identical to the Kiron. Whether newer ones have another optical design, I don't know.
If they are dedicated for APS-C sensor format or even AF, they may.
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