Visa's for SE Asia

Hello everybody!
I am currently planning my 3/4 month trip around SE Asia. I fly out at the end of this month and hold a UK passport. I plan to visit Thailand, Laos, Vietnam and Cambodia. I am aware of all of the visa restrictions I think, but just wanted to double check a few things.

Thailand: I will be flying in, so therefore will recieve a 30 day visa exempt stamp. I will provide evidence (a flight ticket) to show that I will be leaving Thailand within 30 days as I'm aware sometimes the airline require this. I know some of you will say not to worry, but I would rather have one to be sure.
I plan to go in and out of Thailand a few times throughout my trip and have read this should not be a problem. (15 days overland I know)

Vietnam: I attempted to purchase a "visa on arrival" form, which enables me to get a 30 day stamp when I arrive (if arriving by air). Until I started filling in the form I was unaware that you have to predict your date of arrival?! I'm not even sure at the moment which month I will be arriving so wouldn't want to guess and then waste days. (As I know the 30 days start from the date you put down, NOT the date you arrive.)
Would it be best to apply over the internet for one of these forms once I'm alredy travelling? When I'm in Cambodia for instance? Seems straight forward to do in an internet cafe, for example?

Pretty sure I don't need to do any preparations before entering Laos or Cambodia?
Although, I will be entering over land so does this limit my days?? And what are the costs of the VOA for these countries?

Feel free to correct me if I'm wrong with ANY of this information.
Thanks. x