General Synonyms:
* Indistinctly
* Unclearly
* Ambiguously
* Fuzzy
* Dimly
* Slightly
* Faintly
* Remotely
* In a hazy manner
Synonyms emphasizing uncertainty:
* Doubtfully
* Uncertainly
* Unsurely
* Ambiguously
Synonyms emphasizing lack of detail:
* Generally
* Broadly
* In a general way
More specific synonyms:
* Sketchily (lacking detail)
* Superficially (on the surface)
* Haltingly (hesitantly)
* Tentatively (with caution)
The best synonym for "vaguely" will depend on the specific meaning you want to convey in your sentence.