@Dimi*****
you get the form at the consulate when you're there.
As far as documentation you need
the original marriage certificate (if you're married in thailand it's the one with the rose border)
Copies of
front&back of the original marriage certificate
front&back of your wife's thai i/d
your wifes house book listing
data page of your passport
visa/entry stamp to Vietnam
You also need a hand written letter from your wife to the consulate asking that they issue you the visa.
Those things a filled out form and the require passport sized photo plus 200USD get you the year-long, multi-entry Non-O visa.