If you have a plan to spend 3 weeks in Vietnam, you can refer to Lindsay Buckley's budget when she traveled in Vietnam three years ago. Lindsay Buckley is the photographer and travel blogger behind Frugal Frolicker.
Money-wise, Lindsay Buckley did very well in Vietnam. She spent about $712 US in 22 days in Vietnam (excluding transportation to/from the country and the items she had custom-made in Hoi An), which equates to about $32/day in Vietnam: this includes food, accommodation, transport, visa, an absurd amount of coffee, and any extras.
If you exclude transportation and visa costs (i.e. looking at just how much it costs, on average, to spend *a day* in Vietnam, not going anywhere), it breaks down to about $29/day. This ended up being on par with what I spent in Cambodia and Northern Thailand, and significantly less than what she spent in the Thai islands.
About 13% ($96) of her total expenses went toward transportation costs, which shows just how inexpensive it is to get around Vietnam by bus. About 60% of my expenditure went toward food, drink, and accommodation (sometimes guesthouses included meals, so it’s hard to break these expenses apart).
The only thing that was exorbitantly priced in Vietnam was the visa: She paid $65 for a 30-day Vietnam visa in Cambodia. Between the recent visa price hike in 2013 and the fact that you cannot obtain this visa at the border, she can’t help but think Vietnam is trying to curb their tourism. Intuitively this doesn’t seem to be the case, as the Vietnamese are so obviously capitalizing on foreigners who come to stay at their guesthouses, book their tours, and buy their goods. But the number of foreign visitors has dropped 5% since the Vietnam visa about doubled in price at the start of 2013, so there may be a correlation there.