Is it true that Lord Krishna is still performing Rasa Lila in Vrindavana, and that anyone who sees it doesn't live to talk about it?
It is true that Lord Krishna is continually engaged in His rasa-lila pastimes (dancing at night with His purest devotees, the gopis - transcendental cowherd girls) in Vrindavana. This is going on eternally. Actually, in order to experience the rasa-lila at all, ones eyes must be anointed with the salve of Love of God (Brahma-samhita, 5.38). Until one has pure devotion to God, no one can witness the rasa-lila anyway.
However, it is said, in Vrindavana by the local people that in Seva Kunj, the place where the rasa-lila takes place and where there are many, many monkeys, that if one tries to hide in the bushes to watch that most sacred rasa-lila without proper qualification then the monkeys will harm them, perhaps kill them, and/or they will go mad. I've never tried it personally, so I'm not able to attest to the validity of this statement.
But I do know that in the shastra (scripture) it is explained that with eyes tinged with mundane material desire one cannot see Krishna at all, what to speak of His transcendental Rasa dance with His beloved devotees.