Don't worry: it's vague for native speakers too! Actually, the vagueness here isn't really the fault of the passive voice. The simple present tense is the real issue here.
To illustrate, let's use your quoted sentence:
History is written by the victors.
This contrasts with the active voice:
The victors write history.
Both of these mean exactly the same thing. Both of them are vague on the things you ask about: When was it written? Who's reading it? The only thing that the passive voice does is put the focus on "history" more than on "the victors". That's because the vagueness isn't the passive voice's fault.
The simple present can mean several different things. It can be a statement of general fact, which is what it means here. It can describe habitual actions. It can sometimes describe ongoing actions (although that's normally the job of the present continuous).
Now, back to your original example:
A letter is written.
This isn't vague because of the passive voice. It's vague because it doesn't say much. It just states (as a general fact) that a letter exists, that someone wrote. The sky is blue, water is wet, and a letter is written.