What nationality are you?

Malaysia usually before best but all are possible from BKK.
Check your consular advisories travel.state.gov from the US
for each of your countries.

beaware the many Visa scammers here and there just DIY.

There is a lot of info you haven't given us, but here is a general outline. If you can provide more info (your nationality, how you are entering each country and how long you are staying in each) we can offer better information.

If you are from the US, you only need a Thai visa if you are staying for more than a month continuously (if you enter overland, you only have 15 days). No visas required for Malaysia or Singapore unless you are staying a very long time (6 months). You must have a visa in advance or a pre-arranged visa for VN. You can get visas on arrival for Cambodia and Laos if you are flying in. If you are arriving overland, you can get them at some border crossings, not at others.

I am from the UK and am White British, as is my partner.
We will be flying into Bangkok (we are currently waiting for our Thai Visas to be approved and sent)then travelling to the places mentioned before.
We will either be entering via plane or land we are not sure yet.
Thanks