score:9
You have a refusal because the ECO did not appear to examine evidence that was favourable to you. When your bundle was returned, the relevant documents were missing.
Last February there was an advisory stating that VFS personnel in south Asia, (viz Pakistan and India) were removing evidence. I don't know if this is happening in Vietnam or not, but it's worth taking note of it. This was also discussed in chat several times. And I put a snippet into META about it. It reconstructs a question I raised when I took a course in Schengen earlier in 2016...
Source: https://travel.meta.stackexchange.com/a/3709/1737
What this amalgamates to is that immediate representations are called for. The caveat is that while this has been observed for both India and Pakistan, there are no such observations for Vietnam at the moment.
If you think this has happened to you, then you should make urgent and emphatic representations; instructing a solicitor to handle it is the 'golden' solution, but nothing prevents an individual from engaging both the VFS and UKVI directly.
Also, they have a habit of not returning evidence that is photocopied or printed out from the net. So if your stuff was not originals this could also be the case. Also they will not return evidence that is forged or deceptive but hopefully this does not apply to you.