I've read your post and had a look at your 3 example pictures.
The very first thing to say is that I am NOT an expert on negfix8

And I also very rarely use colour film at all, so when I wrote my "HowTo" it was really aimed at black and white shooters. I'm sorry if I didn't make that clear, so maybe I had better revise it.
I've kind of given up on negfix8 for the small amount of colour I do shoot and just use Silverfast in its own regular mode, because I've had variable results.
The guy who wrote it is probably the best possible person to advise you, but I will do my best. I have seen many of his colour photos for which he used negfix8 and they seem beautiful to me, but as I say I've had problems sometimes.
So far as I can tell, there are three things to comment on:
1. Is there a problem with Kodak ColorPlus 200 ?
It isn't the best colour film in the world, so it won't be as sharp, fine-grained, saturated or contrasty as something like Ektar. But it isn't the worst either, and I'm sure it's fine for general photography so long as you accept its limitations. I suppose it was designed as a "snapshot" film.
Also remember that film is not designed to be scanned. It is designed to be printed in a darkroom on paper. So the results of scanning film will always be a bit of a compromise - especially because of something called GRAIN ALIASING. I'll leave you to google that and read up on it!
2. Is there a problem with Vuescan plus negfix8 vs Epson scanning software?
Well it's really hard to say. I don't know what it is you don't like about the negfix8 picture vs the Epson picture!
Obviously the colour balance is a bit different, and there seems to be a bit less contrast, but both versions seem OK to me -- because of course I was not there when you took the picture, so I don't know which one is the more accurate version.
Also I don't know what the Epson software is doing as far as sharpening, curves and so on is concerned, and I don't know what editing (if any at all) you did to the negfix8 version afterwards either.
I think it's important to make sure that Vuescan is not applying ANYTHING to the scan and is just producing raw scans if you are using negfix8. I don't like Vuescan, I think the interface is ugly and confusing and it's really easy to leave some settings switched on when you think they are turned off. So be careful with that.
3. Orange mask pixels & negative vs positive
I think you are right, the way it is written is a bit confusing.
The latest version of the HowTo for negfix8 is here
https://sites.google.com/site/negfix/howto
You scan them as NEGATIVES - don't scan them as slides/transparencies.
OK a bit of education (sorry if you know this already) -
When you look at a colour negative, all the unexposed areas, and some of the shadow areas in the actual frame, are coloured orange, and this is what is called the "mask". Obviously this is pretty different to normal black & white negative film.
The orange mask is there to help make the colours print correctly on darkroom colour printing paper. The details of how this works are too complicated to explain here and I'm not sure I properly understand how it actually works myself, but just hold onto that for a moment.
The trouble is, if you just scan a colour negative and then "invert" the colours (you can do this in PhotoShop or whatever), you get very weird results (try it!). This is because the orange mask interferes with how the colours are rendered,
unless you do something in the software to allow for the orange mask, and allow the colours to render correctly.
In order to do this, negfix8 needs to "see" some of the orange mask without any other colours, and best way to do this is to include part of what is called the "rebate" - the unexposed areas of the film around the actual frame.
So here's a scan that includes a few pixels of rebate at the left hand side showing the orange mask :
20150807-Test1-Mask-001-2 by
_loupe, on Flickr
I'll leave you to try and see what difference this makes (as Flickr uploading has just died on me otherwise I would post a couple of examples myself)
Also remember that the inverted scan from negfix8 is not "the finished article" - it will always need some adjustments, almost always some extra contrast and some exposure adjustments, sometimes some adjustments to white balance. This is perfectly normal.
Again, if you really need better details on how to scan colour film and process it in negfix8, I do urge you to contact the gentleman who wrote it
https://www.flickr.com/people/jaz99/