Explanation:

 1.  Vientiane is the capital and largest city of Laos, on the banks of the Mekong River near the border with Thailand.

 2.  Hanoi, the capital of Vietnam, is known for its centuries-old architecture and a rich culture with Southeast Asian, Chinese and French influences.

 3.  Ulaanbaatar is the capital of Mongolia. It’s in the Tuul River valley.

4.  Phnom Penh, Cambodia’s busy capital, sits at the junction of the Mekong and Tonlé Sap rivers. It was a hub for both the Khmer Empire and French colonialists.