How would "To the front/vanguard" be translated not literally but metaphorically? Would there even be a difference?
I am by no means educated in Latin, but I have done some self study. Would "Ad Praeventores" be anywhere close to what I am looking for?
The definition of 'the vanguard' in Latin is actually more commonly idiomatic, as opposed to being a word. If you wanted to say 'to the vanguard', you would actually say 'ad primum agmen', which means to the first [in] the line of march, when taken literally. Similarly, 'novissimum agmen' is the rear, as the newest recruits would march at the back of the army. 'Ad praeventores' would not have any military context, meaning simply 'towards those who proceed'. As to translating metaphorically, I am not entirely sure what you mean - what would be the context of your metaphor?
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)
and Livy:
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.
