Neither of those wars is likely to have been prevented without Ioustinianos.
Roman-Gothic tensions had been heating up for quite some time. During the reign of Anastasios I, for instance, there was a serious crisis over Sirmion that had an excellent chance of leading to war. East-West religious relations were in the toilet too, due in part to the Akakian schism. I can absolutely see a Roman-Gothic war without Ioustinianos.
Roman-Vandal is even easier; the reconquest of Africa had been on the drawing board since at least 468.