Man pages matching name allegro_vertex_decl in section 3alleg5:

ALLEGRO_VERTEX_DECL.3alleg5.en	debian 10 / allegro5-doc
ALLEGRO_VERTEX_DECL.3alleg5.en	debian 11 / allegro5-doc
ALLEGRO_VERTEX_DECL.3alleg5.en	debian 12 / allegro5-doc