I've travelled to Vietnam many times with a business visa. I am certain they keep a record of your visa on their system in correlation to your passport number.
Prior to your departure to Vietnam you should have received a visa approval letter (sample below) stating that your Visa is waiting for you at your port of arrival. This document has your DOB Full, ...
No, not unless your visa is for a duration longer than three months.
To convert your current driving license or an International Driving Permit into a Vietnamese driving license, the applicant must first hold a valid Vietnam permit of at least three month's validity and a current driving license or a valid International Driving Permit.
Yes, this is allowed. The photo then needs to be approved so if it doesn't fit the
requirments it won't pass.
Photo requirements as follows from the GOV website:
1. What photos are required for the application?
Two photos of the applicant must be uploaded:
Portrait photo (4x6cm): Looking straight and without glasses.
Passport data page: ...