I had only a vague impression of where the place was.

我只是大概知道那个地方的位置。

youdao

Now that you remind me of this matter, I have a vague impression of it.

这事经你一提醒， 我脑子里才有点儿影子。

《新英汉大辞典》

I've seen him only once, and therefore have a vague impression on him.

我只见过他一次，对他的印象比较模糊。

youdao

To overturn such an impression would require overwhelming and incontrovertible evidence, not vague recriminations from a few former employees.

FORBES: Masha Gessen Is A Heroic Opposition Journalist, Not a Kremlin Stooge