Do we need Vietnamese visa or will suffice "Approval Letter" from Vietnamese immigration department

Hello,

we will fly to Vietnam with few free hours during layover in Shanghai, and we want to use 72 hour visa-free transit and visit Shanghai.
Do we need Vietnamese visa for fulfillment of conditions to visa-free entry also? Because Vietnamese visa we will received in Vietnam after the presentation "Approval Letter", which is issued by the Vietnamese immigration department.

Other conditions we fulfill:
1) we are citizens of the country, which covers a 72-hour visa-free transit
2) we hold valid passport and airline tickets to a third country with departure within 72 hours
Do we need Vietnamese visa or will suffice "Approval Letter" according to which we get Vietnamese visa?

Thank you for your reply, best regards
Roman