Thanks for your email.
We would like to confirm that Spain citizen are able to stay in Vietnam less than 15 days without Visa, but your previous entry must be far the next entry 30 days at least.
In case you you come back Vietnam within 30 days leaving, you can apply for Visa on arrival here https://www.vietnam-evisa.org/apply-visa.html. After receiving your application we will process your Visa and send you the Visa approval letter via email. Then you print the letter out and use it to get Visa stamp at the Vietnam airport upon arrival.
Please feel free to contact us if you have any question.