Dear Britta Fischer,
Thank you for your question!
Based on your travel schedule, we advise you to apply for 1 month single Vietnam tourist visa. In case you visit Vietnam by air, Vietnam visa on arrival is the best option for you. Please click on here to apply online. If you choose to enter Vietnam by land or cruise, the only way for you is to apply by post or in person at Vietnam Embassy in London.
Hope it is clear and helpful for you.