You need a visa to enter every country that you will be visiting; however, not every visa system is identical. The visas you are able to get depend on the country you will visit, and the country you are from; some visas you can get on arrival, others you must arrange before.
Also, how long you will be able to stay in any given country depends upon your nationality.
Remember when applying for visas, you will need to bring two passport photos for EACH visa…so, if you are backpacking through five different countries, you will need TEN passport photos.
Each visa requires you to pay a fee, which is different in each country; and, depending on the port of entry, you may have to pay a ‘service fee.’ Be sure your passport has atleast six months of validity left.
Vietnam has a tricky visa system because they don’t technically have Visas on Arrival—meaning, you can’t just show up at the airport expecting to get in. There are two primary ways to arrange a Vietnamese visa; you can send your application and passport to the Vietnamese embassy in your country and wait for it to be sent back (this requires a bit of time), or, you can arrange a visa approval letter through a third party company.
There are companies (that you can find online) in Vietnam who you can pay to arrange a Visa for you; for a small service fee they will email you a visa approval letter…once you print this, you can show up at the airport and wait for your visa there.
One company to check out based on travelfish.org recommendation is Vietnam-Visa.com. They have been providing Vietnam Visa services for over 10 years. They also have a section on whether VOA(Visa on Arrival) is available for your Nationality. All the information is available for you there and if you have further questions you can contact them by phone, email or via chat.
You can get a Cambodian visa at most ports of arrival overland, and at each airport. When arriving at the airport, you will need to pay for the visa with USD. Cambodia also recently announced an eVisa system, which allows you to get your Cambodian Visa online (https://www.evisa.gov.kh); you simply fill out the application, and wait for an email with your visa available to print. Of course, you can also arrange your Cambodian visa at the embassy within your country, or another country you are visitng.
As with Cambodia, you can get a Laotian visa at all ports of arrival overland, and at each airport as well. Paying for your visa with USD will get you the best possible rate, though you can also pay in the local currency. You can also arrange your Laotian visa at the embassy within your country, or another country you are visiting.
The availability of Thai visas changes dramatically depending upon your port of entry. If you arrive overland, you will be allowed a different visa than if you arrange it beforehand at an embassy, which will also differ from if you fly in to Bangkok and get your visa at the airport. Double check your port of entry online to confirm what is available. Again, USD offers you the best rate, but Baht is just fine as well.