If you live there a recent utility bill, copy of driver's license, a selfie of you with today's newspaper in front of Buckingham palace 😁, if you are not a resident then your passport entry stamp.
Yes and yes. By we, do you mean I? Check the particular embassy website to make sure they don't have any restrictions on staying in Vietnam once you apply. Have they asked or have you supplied copies of all your passport pages already?
Suggest you apply immediately, a recent post from another Polish citizen told about the embassy requesting passport pages and stamps, the embassy requirements include being in the country for the application submission, but it's unclear if they also want you there during the processing time.