If you are in Bangkok and need to organize a Vietnam visa there, you can do it easily if you know where to go.
At the moment, Vietnam has not joined any agreement on common visa with any country in Southeast Asia region, so, even you got a Thai visa, you still need to organize a private visa to visit Vietnam if your nationality is not included in the Vietnam Visa exemption list.
If a visa is required, you can do it either by visiting an embassy for the visa to be put in your passport, or by applying online prior to arrival and collecting your visa on arrival at the airport. If you don’t have a visa, they won’t let you in.
Get A Vietnam Visa Online
This kind of visa is called visa on arrival. To use this kind of visa, you need to apply online for a visa approval letter (which is sometime referred to as OK to board by Indians J). This letter is issued by the Vietnam Immigration Department. You then will need to present this letter to their airlines to Vietnam to get on board and then show to the immigration officer at Vietnam airport together with other documents and stamping fee to get visa stamped onto passport. It means that you MUST obtain the visa approval letter BEFORE your departure for Vietnam.
And you should notice that this kind of visa is available only if you traveling to Vietnam by AIR.
Finding the Embassy of Vietnam in Bangkok
Alternatively, getting a Vietnam visa in Bangkok is pretty easy. Get a taxi or tuktuk to Wireless Road (Soi Wittayu) in the centre of Bangkok. Walk out through the elevated skywalk and then down the stairs onto Wireless Road. There is a huge dark gray futuristic-looking building on the corner called Park Ventures Ecoplex on the corner of Sukhumvit Road and Wireless Road, so you’ll know you’re in the right place. Walk down Wireless Road towards All Seasons Place, which is a huge mall and office block.
You will need two photos for your Vietnam visa application. If you haven’t got them already, there is a photo shop that can do passport photos in Wave Place, which is the mall on the opposite side of the road from Park Ventures Ecoplex. You can see Wave Place from the Skywalk before you go down on the street.
Walk along Wireless Road for about five minutes. The Vietnamese Embassy is just along here, but it’s easy to miss because you are so close to it. Look for this door, which is the entry to the Visa Application area.
As suggested by Travellucky, as at February 2015, it’s 3000 Thai Baht for a single entry visa if you want it ready next day, or 2400 Baht for processing in 4 days. However, the the Vietnam Embassy tends to change its prices and delivery schedule quite frequently. Don’t be surprised if the prices have gone up or the processing days have changed. Take extra cash with you.
In case you apply for Vietnam visa online, it is US$46 for a 1-month single entry visa (consisting of US$21 as stamping fee to get the letter within 2 working days and US$25 as stamping fee).