I will give you an example of savings:
We are Canadian and getting a 3 month, Multiple entry Visa through the Vietnamese Embassy in Canada costs $121 per person. You must ship your passport by registered mail with a return registered mail envelope to the Vietnamese Embassy in Ontario at a cost of about $15 each way. This equates to a total cost of just under $300 for two people.
Earlier this year we obtained two "Visa on arrival" letters prior to leaving home at a total cost of $28. When we arrived in Vietnam we submitted the visa on arrival letters and $50 each and about 20 minutes later we were through immigration an out on the street in Saigon. Total cost $128.
COST DIFFERENCE $172
Hope this helps.