Visa

When moving to Vietnam to teach, is it better to come prepared with a six month Visa already purchased? Or just a three month and get another three from Thailand or elsewhere??

If you can get a 6 month visa, then do it. One less thing to worry about when you first get here. 3 months will fly by.
That said even with a 3 month visa you can renew it in country without any problems. I forget if its two or three times that you can renew before having to leave the country to renew.