Yes a home office contract can count as remote work. Demonstrate your work online and submit permission to work remotely in Thailand. If applying in Germany they may accept a contract in German, you will need to confirm that with the embassy.
You can submit a client's business registration info. I did so as a freelancer applied in Taiwan though. They seem to want an actual booked ticket with booking ref. You can create a formal employment contract/certificate with your client, I did the same. Having it on letterhead, gives it a more professional, authentic image.
2 weeks should be enough to get approved in Vietnam. Leaving Vietnam before getting approved risks your DTV application being cancelled or the DTV revoked upon issue.