E-visa process possible? Or upon arrival?

Hello!

We intend to visit Vietnam for one week from 22/Dec/2015. We both have Mexican nationality. We plan to travel from Cambodia to Ho Chi Minh City by night bus.
We would like to process the e-visa. Is it possible to process it even when we get into the country by bus?

If not, can we process it upon arrival? We would like to avoid doing it prior to our trip.

Thanks in advance!

Elena Hernandez.