1/CDC Report on Vietnam'sIncredible Tracing/Quarantine Program: after outbreak in major Hanoi Hospital, entire 7,664-persons staff quarantined. Contact tracing in the community resulted in an additional 52,239 people being quarantined. After 3 weeks, outbreak was contained

2/They didn't just quarantine close contacts of infected, but all hospital visitors & their close contacts (F2s) were provided quarantine support: room and board. Contacts of F2s (F3s) & Contacts of F3s (F4s) were even asked to self-isolate: https://wwwnc.cdc.gov/eid/article/27/1/20-2656_article

3/Aggressive tracing/quarantine in Vietnam isn't unique to this outbreak. As @findingsen writes, once case is identified, all contacts in prior week are identified and quarantined. Those individuals' contacts are identified and asked to self-isolate.
https://globalasia.org/v15no3/cover/vietnams-pandemic-success-is-a-lesson-for-the-world_sen-nguyen:

4/Ability to break chain of transmission, combined w/clear & trustworthy public health communication are among major reasons why Vietnam, a country of ~100 Million, has had just 1400 cases and 35 deaths during the entire duration of the pandemic.