。
She has a vacant look on her face.
Literal
She [topic-は] vacantly face [object-を] is-doing.
~顔をする ('do an X face') is the standard Japanese way of describing a facial expression — literally 'do a face,' but really 'wear an expression.' ぼんやり as an adverb modifies する, painting the face as hazy, unfocused, or absent-minded. The state-mimetic ぼんやり covers both visual blur (ぼんやりした景色 'hazy scenery') and mental fog (ぼんやりしている 'spaced out'); here the latter is at work. ~ている marks the resulting state — she's wearing this expression now.