Hope you're all well.
No matter how many times I have Googled, checked forums, I am still none the wiser to this predicament I have:
I have booked return flights to and from Bangkok, 21st July - 28th August (from Manchester). During this stay, I intend on going to Vietnam for 2 weeks. When I then re-enter Thailand, do I need to pay for a visa (UK citizen) or do I just go through immigration as normal?
Thailand gives a free 30 day visa to most Westerners arriving by air - I believe it's 15 days by the land borders
Vietnam on the other hand is much more of a problem - I suggest the VOA visa if you arrive by air (Hanoi much quicker for the VOA than HCMC)
I wrote this off the top of my head - suggest you check on my facts!!!
Johnny is right. Use the visa on arrival option for your VN visa. It is cheaper and easier than buying one in the UK or BKK. My blog covers my experience flying into HCMC, but HaNoi will be similar. Have your arrival letter in hand, 2 visa sized photos, $25.
I am assuming you will fly back into Thailand. You will get another free 30 day entry permit. Just leave before your 30 days are up and no problem. Good luck with your planning.