Caveat is it has to be done as part of facilitating your escape. Either way (and I've not seen the incident so can't comment on the specifics), the pitch invader is engaged in aggravating behaviour in the proximity of Vieira, where he's not allowed to be, and Vieira is stuck on the pitch with an obstructed route to safety (away dressing room). No way does any jury reject a claim that he had reasonable grounds to believe an attack was likely and imminent. As long as it was a single blow, it's not assault in the eyes of the law as far as I understand it.