1. http://vietnamvisapro.com - this is my current agency of choice. They are quick. I usually get my three months multiple entry visas from them but they will also do single entry visas.
2. https://www.myvietnamvisa.com - I use to buy the visa from them from 2007 to 2011. They stoped providing this service for Indian travelers for a while, but looks like they have started it again.
Note that you can only use this online visa when you arrive by air (any of the three airport - Hanoi, Saigon - Ho Chi Minh City, DaNang). If you are coming in by land, contact a travel agent in Thailand, Cambodia or Lao for the visa.
1. You fill in the online form and make the payment via their online payment link.
2. Look out for their email that acknowledges your submission and payment. This should arrive within an hour. Check your spam folder. If you don't see the email, just email them back.
3. The agency will send you an email in a day or two with the invitation letter. Usually the first page is a text acknowledging that the persons listed in the following pages have been granted the arrival visa. The following pages have a list of names, including your name. Print all the pages, even the pages that do not have your name. Most agencies will also include a link to the arrival form. You can print and fill this beforehand to save time. This form is also available at their Visa on Arrival counters.
This is how the immigration area looks like at Saigon's (Ho Chi Minh City) Than Son Nhat Airport. Walk over to the visa stamping counters. They are marked as A in the illustration above. Submit the passport, invitation letter, filled arrival form, photo and the visa fee. They take about 10 to 20 minutes depending on the volume. You can rest at the waiting area (B). They will then call out your name and return the passport and a receipt for your visa fee payment. Walk over to the immigration counters (C) in the figure. Theofficer here with make an entry stamp on your passport. You can then go down the escalators (D) to pick up your bags and walk out after a customs inspection.
The procedure is similar at other airports where this facility is available.
Some other tips for Saigon (Ho Chi Minh City)
Just after the immigration counters there is a currency exchange counter. I have found their rates to be better than the counters outside. They will also sell you a SIM card- though it is easy to find WI FI in Vietnam, I would recommend getting a data SIM card - they are inexpensive.
The best exchange rate in District 1 is at a small shop near the Benh Thanh Market. I have marked the shop in the map below.
Once You are out of the airport, you can take public busses to popular neighborhoods in the city. I always walk to the domestic airport next doors and take a Vinasun or MyLinh taxi from the taxi counter. It is cheaper compared to taking the taxi from the domestic terminal. Taxi is metered and usually costs around 1,20,000 VND to the city. If you are even more adventurous, walk out of the domestic airport to the Parkson Mall across the street. You can negotiate a cyclo (motorbike taxi) from here to the city center for about 60,000 VND.
I will also recommend getting a Uber from the airport. It is easier to get them to come to the International airport just outside the arrivals area. Uber app will ask your for the number on the pillar near your location. Many streets in Vietnam have the same first name, so make sure you are typing in the correct location.
Vietnamese is a tonal language, it is unlikely that a cyclo or taxi driver will understand a foreigner's pronunciation of Vietnamese words. I bring up my destination on Google Map on my phone and take a screen shot. Once the place is stored as an image on your phone, you can zoom in and show the text to the driver. This is a must with cyclo guys as many of them are older gents who can't read small text.
If you are reaching Saigon or Hanoi during the day, and don't mind staying in the backpacker area, don't worry about booking in advance. There are loads of economical hotels and hostels at Pham Ngu Lau in Saigon and near the northern shore of Hoan Kiem Lake in Hanoi. Just ask them to show your the room, walk in the room, check the cleanliness and wifi strength etc and take the room if you like it. It is perfectly fine to politely decline if you do not like the room.
Google map links
Hotels near Hoan Kiem late in Hanoi https://www.google.com.sg/maps/search/hotels+near+Hoàn+Kiếm,+Hanoi,+Vietnamemail@example.com,105.8482498,16z/data=!3m1!4b1?hl=en
Hotels at Pham Ngu Lau in Saigon (Ho Chi Minh City)