Vietnam forum

Hi, i'm new to the forums but i imagine i will be posting a lot in the next few months, as I am due to go to South East Asia for 6 months at the end of October. I am going to Thailand, Laos, Cambodia, Vietnam and then back to Thailand (and depending on how much time I have to Myanmar for a week).

Because i have a lot of time, I was thinking of spending more than 4 weeks in Vietnam, perhaps 5 or even 6 weeks, but i'm not sure about the visa situation. I know I need a visa, which I will get from Phnom Penh (I plan to get to HCMC from Phnom Penh via boat). However, if i am staying for more than 30 days, I will need to get an extension, or renew it. But i'm not sure where I can do this? I know it can be done in Hanoi , but obviously that won't work out because that is where I am flying from to Thailand and would be the end of my trip in Vietnam. Somewhere in between HCMC and Hanoi would be perfect.

Thanks for any advice.