Can someone explain please when vostro (vostri) can be implied (as in this case) and when it must be explicit? I don't quite see the pattern.
By my understanding, the Italian translation of "Keep your(pl) eyes open." would more properly be "Tenete i vostri occhi aperti." or "Tenete aperti i vostri occhi." if "open" is emphasized, but I recognize that people don't seem to speak like this.
Maybe this given translation is idiomatic rather than literal? Much like "Eyes open!" would be a common idiomatic way of saying the same thing in English, where "keep" and "your" are both implied.
With body parts, you generally use the definite article instead of a personal pronoun
That's not the way people would ordinarily say it. You control your eyes, you don't leave them.