The Evisa is pretty easy to do online. It's not a full page visa, just a PDF. But everything is done electronically, so you won't need to show it in Vietnam. Occasionally, the airline will ask for it at checkin.
You definitely don't want to run appliances at 12 or 24v, a pure sine wave inverter is the way to go. I ran a house using solar at 48v through an inverter.
Anything at 12v requires wiring the size of car battery cables.