Ok, you have a volt meter, there is some hope for you and you know how to use it, better yet (grin).
Battery going totally dead like that suddenly is most likely a draw. More likely the vacuum pump stuck on, but other stuff if you give it long enough (radiator fan switch stuck on and such).
After that its a work through it thing.
Get started, check battery when the car is running. It should have 14.5 and maybe as much as 15. Definitely not less than 14 (and if it is usually a lot less). If so, it probably died as soon as the jump was removed.
Low voltage when running, then its the alternator or the regulator (and I don't know if they are together or separate, not being a VW guru.) There are ways to check it out, but you have to have the book or know VW.
Battery charged, stop, turn on all the lights and check voltage. If it reads 12.5 or so when you start, and slowly goes down, its likely (not guaranteed but very likely good). If it rapidly drops below 121 volts, bad battery. Also dying like that can wreck an older battery, but the cause is still there.
Vacuum pumps to run the door locks? Is this fine German engineering? Sheese the last time I saw a vacuum thingy working cab stuff on a vehicle was a 67 Bronco, and I think they went electric in 68. Amazing.