Thanks for your question,
Vietnam offers 15-day visa exemptions with single entry for visitors from Germany. However, your passports should have at least six months of validity upon the date of arrival to meet airlines requirements, your arrival date is NOT within 30 days from your last exit date with visa exemption. Otherwise, you will need a Vietnam visa.
However, you want to stay more than 15 days so you can apply visa at this link https://www.vietnam-evisa.org/apply-visa.html
The processing time takes 1-2 working days for tourist and 3-4 working days for business.
We can process maximum 3 months visa for both.