International phone charges are steep in Vietnam and many hotels, especially up-market ones, add extra fees. It is however very easy to pick up a SIM card virtually anywhere in Vietnam. Be sure to unlock your phone before you come or you will find that the SIM card won't work. Many SIM cards are also 3G.
A suggestion for making international calls is to log onto or sign up for an internet phone provider, like skype, before leaving home. There are also ones that have been recently set up in Vietnam.
Email and interent is readily available in almost every hotel. Although, a times it can be a little tempermental as the servers tend to be overloaded.