Brigitte, you make good points in your response.
Leaving the key 'on' probably contributed to finding the new battery dead. If it is warranted by the seller you're ahead of the game. Otherwise, just slowly bring it back to full charge using the lowest setting (perhaps ~2-amps?) on your charger.
All batteries prefer a trickle charge compared to an intense fast charge that generates heat and gasses.
I have a memory from years ago that some VW's had a "positive ground"---confirm how yours works and make certain how your tractor battery is hooked to the VW battery using your jumper cables. If in doubt, switch the VW bat into the tractor to experiment.
Another consideration: the jumper cables themselves may be presenting a problem. I see a lot of itty-bitty jumper cables smaller than your finger--almost pencil size--that are essentially useless. The wire inside the cable is so small it cannot conduct sufficient current to start a high-compression diesel engine.
Not inexpensive but well worth adding to your arsenal is a set of heavy-duty (may be called 'trucker type', 'commercial', or 'oil field') jumper cables. Try online for better prices than parts houses. Box stores may not have them. Some welding supply companies make up HD jumper cables. You want longer ones, too. Sooner or later you'll have to stretch from behind or two parking slots over. Personally, try for #0 (zero, pronounced 'ought') or #00 (odd nomenclature: smaller number, more zeros in this case, is larger cable) copper fine-strand highly flexible welding-type cable, heavy clamps, 20-ft long.
Assuming your decompression pull knob works as intended (recent posters have described cables inoperative due to age and corrosion)---fully depress the clutch to satisfy the safety switch if present (latching the clutch 'down' may not engage the switch), pull the decomp when starting, let the engine spin up for couple seconds, then push the decomp in. Should light right off. Be sure to give your glow plugs time to heat up first.
Another good point you mention is considering all the safety switches: on one of mine the PTO being engaged will prevent a start. Be certain everything on the machine is properly set up so you can get a start without safety switch complications. If in doubt, find the Owner's Manual.
Concerning the tiller, most folks tend to "ground" the implement when shutting down. This prevents inadvertent non-powered lowering that might hurt somebody or land on a sleeping shop cat.
Please continue to post back concerning your ongoing experiences so we may all learn.