Vilanova, who replaced Pep Guardiola at the Nou Camp this summer, underwent surgery to remove a tumour on a salivary gland in November 2011 and widespread reports in Spain suggest the 44-year-old has had a relapse.
There has been no an official announcement from Barcelona, although they are due to release a statement later on Wednesday.
Barca earlier issued a press release announcing they had suspended their scheduled media activities for the day, but it did not go into details and did not mention Vilanova.
The statement read: "Barcelona announce that it is has suspended the press conference from president Sandro Rosell planned for midday. As well as that, the media Christmas lunch has been cancelled.
"Later today [Wednesday] the club will issue a further release. Until that time we will lot make any announcement about this matter."
Despite there being no official confirmation about the condition of Vilanova, messages of support from leading figures in sport have been flooding in for the Barca boss - including from title rivals Madrid.
A statement on the Madrid website read: "Real Madrid want to express their support and best wishes for the coach of FC Barcelona, Tito Vilanova, and wish him a very rapid recovery. Real Madrid offer their support to his club and to all his family."
Fellow La Liga clubs and a host of footballers also offered their support.
Manchester City and Spain midfielder David Silva tweeted: "All my support and best wishes to Tito and all of the people in the world fighting every day against cancer. Be strong #AnimoTito."
Liverpool midfielder Nuri Sahin, who is on loan from Real Madrid, tweeted: "Best wishes to Tito Vilanova. I hope he recovers soon."
Athletic Bilbao midfielder Ander Herrera was quoted as saying in AS: "I'm stunned. Is it true? Tito has shown himself to be a fighter, overcoming a very serious illness and I hope that it's going to be the same this time. He has all of my support and that of the club.''
Real Betis said on their official twitter feed: "@RBetisOficial and their professionals offer their total support for the coach of @FCBarcelona_es, Tito Vilanova."