The woodwork twice denied the hosts in the dying seconds through Januzaj and Vermijl's deflected cross with Simpson somehow saving the rebound from van Velzen.
Joyce's side have a 100 per cent record in the competition and are looking in good shape to reclaim the trophy this year.
United: Johnstone; Veseli, Fornasier, Brown (Thorpe 46), Vermijl; Cole, Ekangamene (Pearson 67), Tunnicliffe; Lingard; Januzaj; Cofie (van Velzen 61). Subs not used: Sutherland, Weir.
More: Under-21 fixtures and results | Southampton U21 1 United U21 3