In September 1991, Nicollier arrived in Vukovar with the last volunteers. Nicollier fought across the Vukovar battlefields and was wounded twice in the three months he was there. On 9 November 1991 he was wounded in the leg by a fragmentation grenade, and was transported to the Vukovar hospital where he was treated for his injuries. Nicollier stayed in the Vukovar hospital until the fall of Vukovar on 18 November 1991.