Dear Marianna Friga,
Thanks for your concern,
As you stay is 31 calendar days please applt for 3 months single entry visa type.
It's take 5 working days to process Please apply at https://www.vietnam-evisa.org/apply-visa.html
Apply visa online, You will get visa stamp on the passport upon arrival, All you need to do is fill your
information in the form and make payment online.
After that, we will send You the approval letter via email, please print it out to board the plane and use
it to get visa stamp on the passport upon arrival.
For any further details, please contact us at email@example.com or call us at +84.327.117.119
Stepfano Tran (Mr.)