In regards to your health issue, did you submit the treatment plan with booked appointments or did you focus on past appointments? Did you have the hospital write you a letter?
From their perspective, it looks like you were trying to apply as a digital nomad with paperwork for a company that no longer exists. You only informed them of this when they asked for additional documentation. I could see that as being a major red flag for them. In regards to Vietnam, each embassy has their own requirements. However, it is a central system so your rejection might make it harder to apply in other locations
Kind of hard to tell without seeing what they shared with him. In the many other examples of people’s applications being canceled, they often just say because of a mismatch, so did they really mention the “-“or was that the poster’s assumption. He also says some very vague statements about it not being the same as that on his passport.