I’m a Vietnamese citizen, but since I don’t live in Vietnam, I applied as a foreigner. For the "document indicating current location" section, I submitted a hotel booking for where I’m staying. One person can submit the application on behalf of everyone, but each person still needs to fill out their own form, attach a photo, provide proof of relationship to the main applicant, and pay the $340 fee per person.
Yes, I believe so. I used USD. I'm not entirely sure what exchange rate they rely on, so I checked it on Google and added a little extra, just to be on the safe side.
I think can combine. You just need the total amount in all bank accounts at the time of application submission to be 500k, and that will be sufficient.