Dear David Taplin,
Thanks for your question
We would like to confirm that if you apply for 1 month single entry visa, you are able to stay for 30 days in Vietnam.
Please kindly check this link to apply for a visa on arrival online https://www.vietnam-evisa.org/apply-visa.html
Any more question, feel free to email us at firstname.lastname@example.org
Amanda - Evisa team