Bangkok is always busy!
http://www.mfa.go.th/web/2381.php#Visa
I am sure you have found all the info above already- but I have added it just in case.
Note that you need a photo with you. I think I would sort it out in advance. You will of course have to do this for your Vietnam visa.

Thank you Fiona for your response.

I was looking at different website and the fee on there was only 300 bath so obviously it was old. I think that it will be better if I just go to the Thai Embassy in advance.
We already have visa to Vietnam. The travel agent sorted it out and we will get visa on arrival.
We have just a month to go.. I can't wait

What do you mean by visa on arrival in Vietnam? I didn't think that was possible??

We booked our tour of Vietnam through THG and were told that we can pick up our visa on arrival. I think that if I was going to Vietnam without a travel company, I would have to apply for Visa in advance.
Last week we got our documents, and enclosed was a letter from (I think) Vietnamese embassy with our details which states that we are allowed entry and our visa will be picked up on the date of arrival in Hanoi. We just have to bring a photo and USD 25 each.
I hope that everything will be OK.

That sounds ok! I was just wondering, as we had to apply in advance.