Do I need to be in Vietnam to apply for a DTV visa at HCMC as a German citizen?

Not only must you be in Vietnam to apply at HCMC but you must be in an area of Vietnam served by that consulate.

Short answer: Yes

Did you know, there is a website to apply for a visa

Did you look at it, or just start to ask what you have to fill in at the second question?

Do you need a nanny for the next 10 questions?

You have to be in the country of the embassy you're applying from

You must be in the country and the jurisdiction covered by the embassy you're applying in. You've been in the group for 5 months, there must have been hundreds of posts related to where you must be while you're applying.