It depends on your nationality (some may not need), visa type, how long you stay, normal processing or urgent processing... As I researched, there are two kinds of tourist visa for 1 and 3 months and you can apply also for single entry or multiple entry.
Once arrived in Vietnam, tourists can also apply for Visa Extension of
Tourist Visa if they want to spend more time here. Stamping fee is the
fee you pay the Immigration Officers directly and in cash (USD or VND)
at your arrival airport in Vietnam (Immigration Desk/Counter) to have
Visa stamped onto your passport. The fee is collected by Immigration
Officers and compulsory for all those who use Visa on Arrival method.
For 1 month single entry, stamping fee is $25/person
For 1 month multiple entry, stamping fee is $50/person
The rate is displayed here (Vietnam Visa Fee). Nowadays, it's easy to apply a Visa on Arrival online (You will need to pay extra service fee, for $2 to $7).