Vi coactus is a Latin term, abbreviated as V.C., and means: 'under constraint'. It is used to indicate an agreement made under duress.
An example of its usage is that of the Dutch 17th century statesman Cornelius de Witt, who was forced to sign the act for restoration of Stadholderate. After all the entreaties by his wife, though he signed the contract but only before adding V.C. to his signature.  In the early chapters of his historically set novel The Black Tulip, Alexandre Dumas describes the episode in detail, along with the role and meaning of the two letters V.C.