"First of all, I think that's kind of below the belt and something that should be kept private," Dorfman, 27, coldly told her runner-up. "The things I told you were real. The way I kissed you, those things were real."
The sales account executive shot back that "that was fiancé-type stuff," and tensions continued to grow until host Chris Harrison stepped in to end the segment.
Despite his unexpected reveal, Viall insists to PEOPLE he's ready to close the book on his romance with Dorfman but admits he's not completely over the relationship.
"I've accepted her decision very much, absolutely," he says. "But I think when you love someone as much as I loved her, I don't know if you totally get over that in 10 weeks. I'm still working through it."
"Completely blindsided" by Dorfman's appearance at his door to tell him she was in love with someone else, Viall still finds it hard to believe he wasn't the one to end up on bended knee.
"It's hard to misread some [little things]," he explains. "A lot of that stuff off camera … [like] when you tell someone you love them and they say 'I wish I could say things back' – you're not supposed to assume things, but there's not much room for interpretation."
Though he didn't get his happy ending, Viall insists Dorfman and her new fiancé, Josh Murray, "deserve all the happiness that comes their way."
"Josh for much of the time was a very close friend of mine in the house. He's a great guy and he has a wonderful heart, and I think he's someone that will not take for granted just how special of a person Andi is," he says.
Viall hasn't stepped back into the dating pool quite yet, but he says he's still open to finding love even after getting his heart broken. "I'm 33, and I feel incredibly lucky to have the life and friends and family that I have … I'm patiently waiting to find that person that feels the same about me. I'm looking forward to getting there."