You do not need a letter when getting visa in your home country, it can help speed things up a little but it isn't needed. Here is what the US website says
How To Apply For A VISA To Vietnam
I/ A VISA to Vietnam can be applied for by mail or in person at the Consulate General of Vietnam as early as six months prior to the date of travel. The applicant must submit:
1. Original PASSPORT (which requires at least 01 month validity prior the date of exit from Vietnam and/or 06 month validity to meet airlines聮 requirements). In urgent cases, a loose-leaf-visa can be requested, then a copy of passport (photo and personal detail pages) may be submitted instead of the original passport.
2. Application form (PDF Form/Word Form)The form must be completed, signed and attached with 01 original photo (2x 2in) . In case of a loose-leaf-visa request, 01 additional photo must be stapled to the form. If the applicant has already got A visa approval, she/he may write the reference number on the top of the form. For anyone who comes to Vietnam for less than 3 months and does not have visa approval, just leave 聯visa approval number聰 blank and the Consulate General will take care of it. Additional processing fees for visa approval may be required.
3. Visa fees must be in the form of MONEY ORDER, or CASHIER聮S CHECK, or CERTIFIED CHECK payable to "THE CONSULATE GENERAL OF VIETNAM." Applicants applying together may submit combined payment in one money order or certified check. Fee for stamping visa can be checked at Consular Fee. Other current fees (such as visa approval arrangement fee, processing fee, shipping, handling and expedite services) may be checked at:
Email: vietnamesevisa@gmail.com or visa@vietnamconsulateinhouston.org
Tel: 713-850-1233 or 713-840-0096
4. A prepaid return envelope, if the applicant requests the visa be returned by mail. Please use USPS - United States Postal Service (prepaid postage stamp envelope or return label bought online at ) or prepaid return FedEx label (bought online at - The Consulate General does not accept credit card for FedEx mailing service). For your documents safety, the Consulate General advices to use USPS Express Mail or FedEx with the tracking numbers. Applicants applying together may provide one envelope for visas to be returned to the same address. For international shipping, please pay in the form of money order payable to the Consulate General of Vietnam.
Diplomatic and official visas will be granted GRATIS unless otherwise agreed upon between Vietnam and the applicant's country. In addition to the documents specified in (I), the applicant must submit a note verbal or an official letter from the relevant agencies of the US Government, Embassies or Consulates accredited to the United States, and international organizations.
II/ Processing Time: Five business days are required to process visa applications with approval. Expedited processing requires one to two business days after the Embassy has received the application (additional fee required).
Note: Please keep the tracking numbers of the envelopes to and from the Embassy. When the above deadlines have passed, use the tracking number to check with the mailing service first.
Please do not make telephone inquires on the status of your application (make sure that you keep your passport number for this request) until after the 3rd business day for an expedited case and the 7th business day for a regular case.
The Consulate General will be closed on Saturday, Sunday. Please take account of the observed holidays in planning for your visa application and travel. In addition, no mail can be delivered at the Consulate General during weekends and holidays.