If the Iranian passport holders want to apply visa at Vietnam Embassy, they should to know that:
Visa administrative Institution: Vietnam Embassy
Processing time: depends on you how fast you need. But for sure, you should make a call to Vietnam Embassy for details.
Required Documents: passport, money and 1 new envelope with stamp on it and your exact home address in order to avoid losing issues.
How to apply: please take a look at contact information of Vietnam Embassy below
EMBASSY OF VIETNAM IN IRAN
Address: No. 54, Corner of Borzou St., Ejazi St. (Assef), Zaferaniyeh, Tehran
Phone: +98 21 22411670
Fax: +98 21 22416045