The Argentine is in Valencia today and was undergoing a medical in the afternoon. He will sign on-loan to the end of the season.
Vietto admits the presence of Valencia coach Marcelino had a big influence on his choice.
"I'm very happy, I really wanted to get to Valencia, I wanted to come," said Vietto earlier on Wednesday.
"I hope I can give the best to his team (in reference to Marcelino) and improve it too."
Valencia moved late to trump Sporting CP over Christmas to secure Vietto's commitment.