A letter is written.
The passive voice is not familiar with me and so vague to me. I would think that the sentence above means: “A letter [i] has been written and [ii] is seen by the speaker now.” Is it what the sentence mean? Can any other meanings be there: for example, a letter is being written now, etc. - it seems at least part of it to be in : It is often said that history is written by the victors (daum.net)?
A simple present passive has the same range of uses as a simple present active:
To express enduring truths, as in your second example. History is written by the victors means, approximately, It is always the victors who write the Received History of a conflict.
To describe repeated or habitual actions over time. A letter is written every 30 seconds to the White House.
To report events as current 'hot news', as in a sportscast or photo caption, or by convention in chronicles and plot synopses. In Chapter IV a letter is written, which will have unfortunate consequences in Chapter XII.
With future reference in a schedule or timetable. On Monday a letter is written. On Wednesday the Board receives it. On Friday the Board convenes to discuss it.
With contingent future reference in subordinate clauses: If a letter is written I will answer it. I'm doing it now, before a letter is written forbidding it. Keep any letter which is written you on this subject. Let me know when a letter is written. It is essential that a letter is written.
But without further context it is impossible to say which of these uses obtains in any particular instance.
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.
Another possibility is that 'A letter is written' is an example of what is called 'the historical present'. Sometimes people use present tenses to tell a story which would otherwise be told in past tenses, to make it more interesting and - well - 'present'. It happens with many jokes - "A man walks into a bar and orders ...' - and when summarising a novel or play - "Romeo loves Rosaline but meets Juliet at a party at her house and falls madly in love with her". One sentence of four words isn't enough to tell whether that's the intention here.
