Kubo drew the designs for the characters for continuity purposes but did not participate in constructing the story; therefore, the story in of itself is not canon. If it was, Then white Ichigo (the real Zangetsu) would have appeared when Muramasa pulled Ichigo's Zanpakto out instead of the thousand year younger Yhwach. With your logic, Hellverse is canon (Gates of hell are canon) and Zangetsu is stated to be able to destroy the chains of hell which even Yamamoto could not comprehend / belive was possible. Not to mention he was about to destroy the whole world (stated over and over again). And yes, vasto lorde ichigo IS ZANGETSU. Zangetsu simply waits until Ichigo is at his weakest and takes over. Using basic logic, Zangetsu being fused with the young Yhwach is logically stronger than vasto lorde since its adding another source of power on top of vasto lorde. I understand it isn't a perfect comparison because Kubo did help write the hellverse movie and is deemed canon material, but I'm trying to explain why just one thing out of a filler part being used as cannon doesn't make the entire filler part canon. Anyways, Zangetsu is able to use a getsuga infused gran ray cero. He has all of the abilities Ichigo had at the end of the series because they are basically the same thing. FOR THIS REASON, my vote goes for Zangetsu.