Vin Diesel -- the voice of the resurrected plant hero -- really got into the shrubby spirit Monday at the "Avengers: Infinity War" premiere in Hollywood ... wearing part of Groot on his suit!

The premiere was insane -- and even had a special cameo from Stan Lee ... who's been battling health and legal issues. He got a hero's welcome.

Other stars include ... Chadwick Boseman, Scarlett Johansson, Robert Downey Jr., Chris Hemsworth, Gwyneth Paltrow and Star-Lord himself, Chris Pratt.

The one star who didn't make the premiere -- Chris Evans. Guessing it has to do with the fact he's starring in a play on Broadway at the moment.

As for the reviews ... so far, pretty solid. Kevin Smith LOVED it.

"I am overwhelmed. The movie is epic beyond compare."