I have six month visas in my passport for both Vietnam and Cambodia. My passport has only four empty pages and as I will be flitting around southeast asia before it is no longer useful (six months before expiry) I am considering getting a new passport. What do these governments do when it's your old passport that has your valid visa?.

Whilst I cannot say what they would do in Cambodia or Vientnam, for most countries you simply present the expired passport containing the visa with your new passport. This is certainly the case in UK, Schengen, US, Australia and Thailand.