My bank sent me that letter via email -PDF, you can ask your bank if they will do the same. For the flight, submit the booking of flight to Thailand from the UK. They want to see the actual booking with flight, booking ref no. etc
You can find in the threads loads of posts inc very recently showing Vietnam does approve with non-seasoned deposits. There is not an exact amount of time to keep the money beyond being in your account at time of submission. There are some who are more cautious and season funds for 3 months before applying in Vietnam.
Anonymous participant You can apply via the freelance invoices you have. Taipei embassy who I applied with last Dec, would not accept my new contract starting in Jan. They said they need to see payments from that contract, so I applied with previous freelance contracts.
If you have just started working remotely then you detail that. Approvals have been given for those recently employed, There is no minimum income requirement, inconsistent income is part of working freelance so it is not an issue to show irregular payments, Key financial is the 500K THB, if you have more in savings show that to strengthen your application,