No. The prosecution specifically tried to tie him to a group (the primary definition of vigilante) and failed. He was proven, on many videos, to be out there "protecting" (essentially a deterant), offering first aid, and extinguishing fires. He didn't try arrest anyone or use any force whatsoever to enforce any laws on his own. He used force in self defense and gave himself up to law enforcement.
IANAL either, but I would think that vigilante is a fairly generic adjective, and would be difficult to win a defamation lawsuit objecting to the usage. Tying him to a specific group or accusing him of a specific act which is objectively true or not true would seem to be a more risky proposition, but I can't imagine him getting a judgement for not being happy with the terminology used to describe him.
Definition of vigilante
: a member of a volunteer committee organized to suppress and punish crime summarily (as when the processes of law are viewed as inadequate)
broadly : a self-appointed doer of justice