Vilanova, who replaced Pep Guardiola this summer, underwent surgery on a salivary gland last November.
Media reports in Spain suggest the 44-year-old Vilanova has suffered a relapse.
Barca released a press statement on Wednesday morning announcing they had suspended all activities for Wednesday but did not go into details and did not mention Vilanova.
The club confirmed a further announcement would be made later on Wednesday.
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 the club will issue a further release. Until that time we will not make any announcement about this matter.
"We regret any inconvenience this decision may cause."
It was announced in April that Vilanova would take over as Barca coach following Guardiola's decision to step down at the end of the season and take a year-long sabbatical from the game.
Vilanova was Guardiola's assistant during four hugely successful years at the Nou Camp which saw Barca win 13 trophies, including two Champions League crowns and three Primera Division titles.
Vilanova has enjoyed a superb start to his tenure in the hotseat, helping Barca establish a nine-point lead at the top of the league standings after winning 15 of their opening 16 games while drawing the other.
Barca are also through to the knockout stages of the Champions League and hold a 2-0 first-leg lead over Cordoba in their Copa del Rey last-16 tie.
Despite there being no official confirmation about the condition of Vilanova, there have been messages of support from leading figures in Spanish sport.
Tennis star Rafael Nadal tweeted: "All my strength and support to Tito Vilanova! We are with you to overcome this new step."
Real Betis said on their official twitter feed: "@RBetisOficial and their professionals offer their total support for the coach of @FCBarcelona_es, Tito Vilanova."
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."
MotoGP world champion Jorge Lorenzo, meanwhile, tweeted: "#AnimoTito (encouragement for Tito)."
It is not known when Barcelona are expected to issue a new statement but the first-team squad are scheduled to train this evening at 1800 local time.
Barca are away to Real Valladolid in La Liga on Saturday for their final fixture before the winter break.