not getting to a Vietnam airport

Hi,

I'm going on a backpacking trip to Southeast Asia, Vietnam included. I will arrive to Bangkok airport and I plan to enter Vietnam on a bus that goes from Vientiane (Laos) to Hanoi around June the 20th, though we don't have a specific date yet as we want to keep our schedule flexible.

If we are not arriving to Vietnam by airport how does the Visa process work?

Is there a way that we could already have our Visa (1 entry), before we leave Mexico at the end of May?

Thank you