No seasoning requirement in Vietnam.All the docs mentioned work for freelance approval. I submitted them all except invoices as I don't invoice. Owning your business you can add invoices
You will be fine via the Medical option of the DTV. Submit 6 -12 months treatment plan from your hospital/clinic. Check the threads for posts of similar applicants for tips.
It is ok to apply from Japan but you need to research in detail what they want not just what it says on the website. It is among the top most difficult embassies to gain DTV approval. Focos on showing long term use of the DTV to avoid getting an METV.
Your age is irrelevant plenty of 50 -70s have got the DTV, some switching from retirement visa. New businesses with no income have been approved for the DTV. You should show how you will support yourself while getting the business off the ground..e.g additional savings. Not necessary to use an agent unless you have probs with submitting paperwork