When you talk about the real Niko I suppose you talk about the one that said he was the real Niko and the one who taught Setsuna the Niko Style and that supposedly lost his life against the other Niko that taught Ohma all he knows currently. Honestly, I don't think that guy is really dead. He seemed to be the cunning and intelligent type judging by how he manipulated Setsuna and the way he slept through Ohma's guard and bestowed to him the Advance. To think that such a guy would just go against the other Niko in a straight-up brawl and lose his life off-screen is quite hard to believe. Something smells fishy here. I think there's something we're missing and that we don't know yet.
I think this Tiger's Vessel as a concept is quite literally. The Worm has been studying brain-related technology, more specifically brain uploading and cyberbrain or something of the sort for quite a time. My theory is that there was a warrior called The Tiger who was extremely powerful but somehow ended up losing his body or his body highly deteriorated to the point that he could no longer continue fighting. The Worm is probably trying to put his brain in another body that is related to the techniques he used (in this case the Niko style most likely, which would be a prerequisite to become a candidate to be his Vessel in the first place) to "revive" him. I think there are multiple options for the identity of this warrior called "the Tiger":
A. Gaoh Mukaku himself. It was said he was unable to fulfil his dream as it took way too long and his body deteriorated a lot in the process.
B. The guy who called himself the real Niko.
C. A completely new character that we haven't seen yet.
I probably haven't answered your question at all, but I thought it would be interesting to comment on this topic