For Thailand, we can get a free visa at Bangkok's airport. These will be a tourist visa for a period of 30 days. Visa that can be extended at the Office of Immigration Bureau in Bangkok but if you stay more than 30 days, I suggest you apply for a visa before leaving your country for peace of mind ( it seems that certain airlines may refuse your boarding if your return ticket is beyond 30 days, there are rumors about this on Gulf Air). Note that if you go to Thailand by land, you will get a visa than 15 days (against 30 days by air), it is a recent disposition to discourage multiple outflows and inflows in the territory dating from June 2009. Since that date the 60 days visa is also free.
For Vietnam, I you really should do it before. Of countries that I'm going through, it is the one that is more complicated and for which delays are longer. Of course, you can also make your visa from a neighboring country but my advice, deal with it before leaving. There are certain travel agencies that offer visas for Vioetnam in Thailand and inflate the price significantly.
Cambodia Visa is simple to obtain at your entry into the country, just prepare 2 passport photos and about $ 25.
Otherwise regarding the vaccinations, are strongly recommended the typhoid and hepatitis A vaccination and to a lower extent the one against Japanese encephalitis. Depending on the zones you will cross the risk of malaria or dengue fever exists. Therefore it's important to protect yourself against mosquitoes and have an anti-malaria treatment. Anyway see your doctor at least one month before departure (to allow time to make vaccines and boosters).