It is unfortunate that your particular VW dealer is unable to perform the necessary troubleshooting to determine the root cause of your discharged battery. Actually, cars are not "lemons" -- it is the dealer service departments and the car maker's technical assistance systems that are the real lemons.
You don't show your location in your profile. I live in central Indiana. If you live close, I have equipment to troubleshoot this kind of problem.
There are 3 basic causes for a dead battery:
1. The battery is defective
2. The alternator (generator) in the vehicle cannot provide a charging current sufficient to keep the battery charged, or
3. There is a parasitic drain on the battery when the engine is off, draining the battery. Any parasitic battery drain can be there constantly or less likely, it occurs only intermittently.
If you have a battery charger (even if it is only a trickle charger), connect the charger to your battery when the car is sitting in the garage to prevent a parasitic drain from discharging your battery. A prolonged low battery SOC (state-of-charge) will shorten the life of the battery.
Did the dealership service manager describe any details of how they determined whether the root cause is item 1, 2, or 3 above? Typically, if the battery can be recharged and when charged it can pass a battery load test, it is likely to be OK.
If the battery has been recharged and the system voltage is 14 volts or higher at any engine RPM above idle speed, the alternator is likely to be OK. There are more sophisticated alternator output checks that can be made if you use a clip-on amp meter to check charging amps out of the alternator.
Troubleshooting a parasitic load on the battery requires a clip-on DC (direct current) amp meter that has resolution to about 0.05 amps. These meters typically use Hall Effect sensors or you can use the more expensive Swain DC amp clips (
http://www.swainmeter.com/index.html). The Swain DC amp clip that I own costs about $800 and is probably not something that the average dealership service department has available to it. These DC clip-on instruments allow you to simply clip over a wire carrying DC current and the current flow in amps can be directly measured by the instrument.
A typical Beetle will have a parasitic current drain on the battery of no more that 0.06 amps (all doors closed and unlocked). This magnitude of current drain would easily allow your car to sit for 30 days without use and still crank and start. However, if the current drain increases to 0.15 amps, your car will likely not start after 11 days of no use if the battery was fully charged when the engine was shut off. Any typical relay that is continuously energized will draw about 0.15 amps. A battery drain of 0.5 amps which is typical of a glove box lamp will drain the battery to the point of a no start in just over 3 days.
Most of the electrical loads in the Beetle are isolated from the battery by either the ignition switch and/or the load reduction relay (relay 109) when the ignition switch is in the OFF position. Therefore, the most likely causes of a run down battery include the modules controlling security, door locking, trunk unlocking, the engine control computer, cooling fans, etc. These are examples of loads that are "awake" when the key is not in the ignition.
Your dealer must be able to measure DC current flow as describe above to isolate the offending electrical load. If a battery drain is detected, fuses can be pulled one-at-a-time to determine the circuit that is drawing the current. If your present dealership service department does not know how to do this, you need to find another one.