Fulham were 2-0 ahead after just six minutes at the Tottenham Hotspur Stadium thanks to goal from Kenny Tete and Harry Wilson.
Vicario, 29, was directly responsible for the second after charging off his line to intercept a pass aimed at Raul Jiminez only for his awful clearance to fall to Wilson, who simply passed the ball into an open net from a tight angle.
Spurs fans made their feelings about the Italian’s performance known throughout the game, loudly booing him and his teammates.
Speaking to Sky Sports after the game, Frank urged the fans to get behind the side rather than boo them.
"When you're down 2-0 after six minutes, there is a mountain to climb. When you're in a bad spell, everything seems to go against you as well - the first was a deflected shot, the second is a mistake from Vic (Guglielmo Vicario),” he said.
“I didn't like that our fans booed at him straight after and a few times after that. They can't be true Tottenham fans because everyone supports each other when you're on the pitch and we do everything we can to perform. I'm fine with them booing after the match, no problem, but not during. That is unacceptable in my opinion.
"After that, we rushed the first half in many ways. We wanted to come back into the game but couldn't do it in the first half, but we got much better in the second half. We got a bit of momentum, created more chances and opportunities, but couldn't get the second goal.
"It was an emotional performance in the second half, and I think that's quite normal. We know that we badly want to win at home, so when it's not going your way, it is hard. That's why you need to stick to the plan, be cool and not stress. It is easier said than done."