Lacking detail (5)
(Other definitions for vague that I've seen before include "Indeterminate" , "Indistinct or not clearly expressed" , "Imprecise (in thought)" , "Uncertain, imprecise" , "Unclearly defined" .)
'lacking detail' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sketchy that I've seen before include "Superficial" , "first draft" , "Incomplete; rough" , "Imprecise" , "lacking detail" .)