To complement the other answer, another common way to refer to the vanguard is prima acies.
In primam aciem is the usual way of saying, "to the van", e.g. Caesar:
scuto ab novissimis uni militi detracto, quod ipse eo sine scuto venerat, in primam aciem processit centurionibusque nominatim appellatis reliquos cohortatus milites signa inferre et manipulos laxare iussit. (Caesar, De Bello Gallico 2.25)
Praenestinus praetor per timorem segnius ex subsidiis suos duxerat in primam aciem. (Livius, Ab Urbe Condita 9.16)
I'm not sure what you mean by "not literally but metaphorically." Acies is used in a myriad of contexts, some of which are decidedly metaphorical. I am sure that a Roman would understand in primam aciem outside of a military context if the context was otherwise clear that it is not meant literally.