E-visa, 3 month multiple entry question.

Hi,
 I am looking at getting the evisa, multiple entry 3 month visa for  myself (British passport). I know that you have to arrive to a main  airport to get the visa once you have arrived in Vietnam, however, if I  have the 3 month multiple entry visa and I firstly enter to an airport and get the visa, am I then able to travel overland to another country and back again to Vietnam overland as long as it’s  within the 3 month validity? Or am I not able to travel overland at all  on the evisas?
 Any help would be greatly appreciated.
 Many thanks,
 Charlotte