If you think you need a starter consider this one. Since you say you can push start it I'm assuming a manual trans.

https://www.amazon.com/Premier-Gear-PG-17066-Professional-Starter/dp/B00KL51S6O/ref=sr_1_4?dchild=1&keywords=vanagon+starter&qid=1625611715&sr=8-4

This starter for the money simply works, and works well. No more adding relays or slow cranking or other nonsense. No adapters needed.

As for trouble shooting you need to look at the symptoms to determine if the problem is the solenoid or motor itself. If the solenoid is clicking you are telling it to go. It could be the resistance of the solenoid circuit is too high for the solenoid to get the gear out and close the contacts for the motor. This is a very common Bosch failure. Again, for under $100 make the uncertainty disappear.

Dennis