「ぼんやり分かる」の「ぼんやり」は英語で vaguely と言います。
vague - 曖昧
分かる- understand
ぼんやり分かる - I vaguely understand もしくは I kinda get it, I kinda understand
kinda = kind of
kinda は砕けた言い方です。
先生の言っていることがぼんやりと理解できた - I vaguely understood what the teacher was saying / I kinda got what the teacher was saying / I kinda understood what the teacher was saying
Example (授業後)
A: Did you understand what Ms ... was saying? ...先生の説明分かった?
B: I kinda got what she was saying/ I kinda understood what she saying. (ぼんやり分かった)