Quos (accusative plural of quis) here is equivalent to aliquos after si.
I should have written "accusative plural of qui
, since quos
here is an indefinite adjective, not a pronoun.
Allen & Greenough sec. 310a: http://www.perseus.tufts.edu/hopper/text?doc=AG+310&fromdoc=Perseus%3Atext%3A1999.04.0001
the logical subject and object are inverted.
Actually, he has endowed an abstract concept, vigilantia
, with agency, and made it the subject of the verb, where you would expect the humans to be the agents and the subject of the verb.
Also worth noting in this line:
1. The interlocking noun-adjective pairs adj1 adj2 noun1 noun2: quos haud ulla uiros uigilantia
2. the interlocking patterning of rhyme and assonance:
at si quos
haud ulla ui
-os ui- -os ui-
Vergil has transmuted a mundane and pedestrian idea into exquisite poetry!