Vietnam and nearly all other countries will never cover up an old visa with a new one, assume it's new real estate in the passport every time. If you hold a US passport, you can get 24 pages added for free when you get low on new pages--and it's easier and quicker to do this at a US Embassy when you are actually overseas, rather than do it in the US. Plan accordingly. For new passports or renewals, best to ask for the 48-page version upfront (no extra charge over the 24 pager) on the application.