Before popping the question to girlfriend Alexis Bledel, the Mad Men actor (who plays Pete Campbell) jokingly practiced his proposal on set with costars Elisabeth Moss and Jessica Paré.
"It was purely comedic but he was giving [Elisabeth] and I a facetious rendition of his planned proposal," Paré told PEOPLE at the season 6 premiere of the hit AMC show Wednesday night. "We were like, 'Great! Lets do this one without cursing and without yelling!' "
All joking aside, Kartheiser's costars could not be happier to hear the news of the engagement to Bledel, who plays Beth Dawes on series, which returns to AMC April 7 (9 p.m. ET).
"They are such a yin and yang," Moss said of the couple. "I've known Vinny for 12 years and Alexis is just the sweetest. I've never seen Vincent like this. It is really cool."
Even Kartheiser's on-screen wife wished the lovebirds well.
"My Twitter feed was abuzz today with [fans] going, 'Oh my God,' " Alison Brie said. "I think everyone thought I was going to be very jealous but I'm excited for him for the record!"
This will be a first marriage for both Bledel, 31, and Kartheiser, 33.