The Catalan giants want a long-term replacement for 34-year-old Carles Puyol and believe Liverpool’s Agger and Arsenal captain Vermaelen fit the bill.
Barca first showed signs of interest in Agger when Manchester City had a £20million bid rejected last summer and with Borussia Dortmund’s Mats Hummels proving too costly, are ready to renew their interest.
Any move would leave Liverpool manager Brendan Rodgers with a project to rebuild his defence. Martin Skrtel has not played well this season and Uruguayan Sebastian Coates has been linked with a move back to South America.
Club stalwart Jamie Carragher retires at the end of this season, and any deal for Agger would potentially mean Rodgers purchasing four central defenders.
Barca also believe a £15m bid for Vermaelen would test Arsenal’s resolve.
If he does leave, Vermaelen would be the third Arsenal captain to leave for Barcelona in six years after Thierry Henry and Cesc Fabregas.