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.
Malaysia is the strictest embassy to gain DTV approval as posts in the threads detail. 21 is old enough to apply. 6 months seasoning of the 500K THB, 6 months accommodation booked in Thailand are examples of what is required in KL.