Vigo offered to serve
George Rogers Clark as a spy during the American Revolutionary War. He was sent to
Vincennes to learn of the conditions at the now British-held
Fort Sackville. On the way, Vigo was captured by
indigenous warriors and turned over to
Lt-Gov Henry Hamilton. Although Vigo was a
Spanish citizen and hostile acts would breach Spanish neutrality, Hamilton suspected his intentions and paroled him on condition of reporting to the fort each day. This actually assisted Vigo's observations, allowing him to assess the fort's strength and defences. Some time later, despite their love for Hamilton, the Canadiens were persuaded by
Father Gibault to demand Vigo's release, on threat of cutting off local supplies to the fort. Hamilton offered that release provided Vigo agreed "not to do any act during the war injurious to the British interests." Vigo accepted on the basis that the terms would apply only during his return to St. Louis. Vigo travelled down the
Wabash,
Ohio, and
Mississippi Rivers to St. Louis. Having fulfilled the agreement precisely, Vigo then turned around and went directly to
Kaskaskia to tell Clark of preparedness at the British fort. Clark took advantage of the information to capture the fort in 1779. A contemporary of Vigo said that he further aided the American capture of the
Northwest Territory by lending his financial backing to the
United States dollar. The Canadiens refused payments in "continental paper" until Vigo guaranteed them. However, the value of the dollar would soon collapse. Vigo reportedly complained that he was never compensated by the US government for supporting American credit during the war. ==Post-war years==