vis

vis is feminine, third declension, and defective: in practice the singular is only vis, vim (accusative) and vi (ablative), because the genitive and dative are avoided in classical prose. The plural is complete and regular for an i-stem: vires, virium, viribus, vires, viribus.

Singular and plural mean different things. The singular is force applied to something, hence violence, and the legal tag vi et armis (by force and arms) is exactly that. The plural vires is strength as a resource: physical powers, and in a military context troops. Two traps. vis is also the second person singular of volo, "you want", spelled identically and separated only by context. And viribus is not viris: vir means "man" and belongs to a different word altogether.