I have been using DXO Photolab and the NIK Collection for about eighteen months but have only just discovered these forums!
I use DXO Photolab 5 as my RAW processor and am very pleased with the results I get. My question is, why would you transfer an image to Viveza to be processed? I realise that Viveza is not a RAW processor, but what can it do that DXO Photolab can’t do?
Generally I agree with @Joanna but Viveza is capable of using Control Points to make local color adjustments to RGB files that IMO is superior to the local Hue adjustment in PL5. The local WB sliders in PL5 are unavailable for non-RAW files. I scan a lot of my old negatives to TIFF files with a cheap little scanner that does not have fine granularity on color balance. While the scanner makes very sharp heavy-weight files, it’s color balance often needs local tweaking and I’ve had better luck with Viveza than with PL5’s lone local Hue slider.
I use Nik Collection mostly because of the filters in Color Efex Pro 4. The rest of the edits I do in ACDSee or PL5. So why not use Viveza when you already have PL5, not a whole lot of reasons. Maybe because you want more control on how the control points work or maybe because you want to use one of the filters provided in that module.