Should I apply for the DTV visa online or go to the embassy in person in Hanoi?

You can only apply online, only go in person if they ask for an interview

you can't "go to an embassy in person" 🙁

Ever since all the thai consulates in the world switched to the eVisa online web-portal, you apply online

So it works like this,

you go to vietnam,

log into the eVisa online system,

apply online,

pay online

then

wait in vietnam until the visa is approved,

once they email you the approval print out the attached PDF file (as that is your visa), and give it along with your passport to the officer when you stamp in to thailand