From Longman Dictionary of Contemporary Englishvacant expression/look/stare etcvacant expression/look/stare etcwrittenTHINK ABOUT# an expression that shows that someone does not seem to be thinking about anything He gazed at me with vacant eyes. → vacant
Examples from the Corpus
vacant expression/look/stare etc• Eyes downcast; baby lips pulled into a frown; dull, vacant stare.• Brittany, the blond cheerleader, has a vacant stare and huge eyelashes.• But an oddly vacant look had come over Cinzia's features.• However, just behind the vacant expression he offered me, I detected fear.• He was looking round with a vacant look on his face and I was frightened.• He was a bright-eyed boy, thin and fair, with a vacant expression that often gave way to shrill laughter.• The maid's vacant expression was replaced momentarily by one of greedy expectation - shortly followed by disappointment. 