Do I need a visa to travel from the mouth of Yunnan to the old street of Vietnam for one day?

You need a visa, but if you only want to stay for one day, you can go to the travel agency in Hekou County and ask them to apply for a pass for you. They will finish it soon.

Visa is the annotation and seal of the sovereign organ of a country on the passport or other travel documents held by its own or foreign citizens to show that they are allowed to enter or leave its own territory or pass through it. It can also be said that it is an endorsement certificate issued to them.

Generally speaking, a visa is a document issued by a country's entry-exit administration (such as immigration bureau or its embassy or consulate abroad) to approve the entry of foreign citizens.

Passport is the holder's nationality and identity certificate, and visa is the permission certificate of a sovereign country to allow foreign citizens or their own citizens to enter, exit or transit.

Visas are generally endorsed on passports, some are endorsed on other travel documents instead of passports, and some are issued separately. For example, immigration visas in the United States and Canada are A4 paper, and Singapore also issues separate paper visas to foreigners. Generally speaking, a visa must be used at the same time as a passport to be valid.