His vacant gaze suggested his mind was preoccupied with far more troubling matters.

gaze 是名词，意思是"凝视、注视"，指长时间专注地看某处的眼神或视线。 vacant gaze 指"空洞的眼神"，形容人虽然睁着眼睛在看，但眼神没有焦点，显得心不在焉或思绪飘远的状态。