Hinard has this Pansa as a tribunicius in 43, with 51 as his year of office. Contra MRR, which has C. Vibius Pansa Caetronianus (cos 43) as the tribune of 51. (Hinard 1985)
According to Appian (BC 3.85), the Senate upon learning of the union of Antony and Lepidus sent these two senators secretly to Brutus in Macedonia to summon him to Italy; but see Cic. Ad Brut. 1.10.1; Dio 46.51.5. (Broughton MRR II)