Vieira expects Watford to sign Peruvian winger Carrillo for around €20m come the end of the season, says O Jogo.
The 26-year-old joined the Hornets on loan at the start of the season and there is believed to be an option to make the deal permanent in the summer.
“It was a big decision for me," he told the Watford website. “I've been on loan five times before now.
“I found it difficult in Oxford when I was younger, being away from home, but as I have got a bit older, being more independent I think has helped me develop as a person.
“I was told Falkirk were interested just before Christmas and actually went up there on Boxing Day to train with them for a couple of days.
“I was impressed with what I saw, enjoyed it and felt like it was somewhere I could come and develop."