Monday’s opening of Christopher Durang’s “Vanya and Sonia and Masha and Spike” at Lincoln Center marked a reunion for the playwright and several actors. The scribe has lost count of the number of collaborations with Sigourney Weaver.

“We first met at Yale in 1971,” Durang said. “We were in the same class and did cabaret shows at Yale. I remember a children’s show where she was an evil baroness and I was her troll. So when she hit me with her cane, I’d smile and look happy.”

“Do we really go back 41 years? That’s older than I am!” Weaver joked.

Just “30 years ago” David Hyde Pierce made his Broadway debut — and got his Equity card — in Durang’s “Beyond Therapy.” “This is the first time I’ve done a production with Chris since then, and it’s actually been very emotional to think about all that’s happened in between,” said the thesp.

Co-star Kristine Nielsen, a showstopper with her impression of Maggie Smith in “California Suite” going to a costume party as Snow White’s Evil Queen, said this was No. 6 with Durang. “I worked with him here in ‘UBU’ (which wasn’t very good) many years ago, and we bonded backstage, surviving that Titanic experience together,” said Nielsen.

As for sending up the dame, Nielsen noted, “My goodness I’m so grateful for ‘Downton Abbey’ because when Chris wrote this the kids only knew her from Hogwarts land” — the Harry Potter series — “and she’s such a massively wonderful actress. I love channeling her.”

Guests at the P.J. Clarke’s afterparty included Nathan Lane, Victor Garber, Frank Rich and Andrea Martin.