This is a typical VW starter problem. The voltage runs from the battery to the terminal on the starter motor, back to the switch and then back again to the solenoid. Front to back to front to back, voltage drop in all of the cables and if there is any corrosion on connection points there wont be enough voltage to pull in the solenoid. Couple things you can do;
1) Clean, clean, clean, any connection point to limit voltage drops and replace any corroded wire.
2) Add an external solenoid wired from the battery right to the starter solenoid to get direct battery voltage to the starter solenoid. A lot of VW places sell these as "hard start" kits.
Hope this helps.
Chris