。
She filled the vase to the brim with water.
Literal
She [topic-は] that vase fully [in-に] water [object-を] put-in.
いっぱい ('full, brimming, completely') is one of those words that lives a double life — as a noun meaning 'a cupful' and as an adverb meaning 'fully'. Here it modifies the verb adverbially: 花瓶いっぱいに ('to-the-vase's-fullness'), with に turning it into a manner adverb. ~を入れる literally 'put X in' is the everyday transitive 'fill / pour'. Note that English 'fill' takes the container as object ('fill the vase with water') while Japanese reverses it: the *water* is the を-marked object that's put *into* the に-marked vase.