In 1385, Vygantas supported his older brother Jogaila by signing the Union of Krewo. On February 18, 1386, he was baptized as "Alexander". In the same year he was betrothed to Jadwiga, daughter of Władysław Opolczyk. He married her in 1390. Polish nobles expected that Władysław would grant Vygantas the Dobrzyń Land, a contested territory between Poland and the Teutonic Knights, as his dowry. Vygantas received Kuyavia from Jogaila instead. In 1388, he supported Poland in an armed conflict with the Teutonic Knights. He also supported Władysław Jagiełło (Jogaila) in the Lithuanian Civil War (1389–1392) against Vytautas. Vygantas was considered as a replacement for unpopular Skirgaila as Grand Duke of Lithuania. However, he soon died in Vilnus. The rumors had it that he was poisoned – either by Vytautas or Skirgaila. He is buried in Vilnius Cathedral together with his brother Karigaila, who died in the civil war in 1390.