You can search for "The Hangover 2009 vietsub" on popular subtitle sites or Vietnamese streaming platforms like FPT Play, VieON, or Zing TV (depending on availability). Some fan-sub groups also offer high-quality .srt files for download.
If you own a digital copy of The Hangover (usually an .MP4 or .MKV file), here is how to sync Vietsub: hangover 1 vietsub