I think of an open-eyed newborn, someone who is enamored with everything s/he sees. By thinking about any word or phrase, we must consider the context.
Whereas, vigilant means keeping careful watch for possible danger or difficulties, on guard, which could lead to being uptight, stressed out, hyper, I think 'open-eyed' could have other emotional qualities like 'innocence."
We say a lot with the eyes, and so vigilant, watchful, or on-guard all have different meaning compared to suspicious conscientious, 'Mind; delirium; open eyes, 'Mind; delusions, imaginations; open eyes, with' or the simplest
dictionary definition: with the eyes open or wide open.
Have you ever asked a child to 'open-wide' and they do this with their mouth or eyes?
Figuratively it can imply clear-sighted; perceptive; fully aware : an open-eyed approach to political manipulation.• with a sense of wonder, awe, etc.