I didn't want to ramble more in my post, but the "smarts" of the vision battery wouldn't be able to handle to amount to current the car battery can provide. The balancing taps and load used to balance the lipo wouldn't be able to use the current that the battery has available, and until the voltage of the lipo and "power supply" (battery or PSU) is matched, current will flow from the one with higher voltage.
If you put a 5a fuse between the batteries, it will just blow when the batteries are connected. The charger on the battery isn't a charger, it's simply a balancer and stops the cells from overcharging, but like all electronics, it will have a current limit it can handle, given the stock charger is 4a, I'd be surprised if the balancer on the battery could handle any more than 8a.
Also, the spade pins on the battery are always in connection with the lipo, there is no way for the battery to disconnect the terminals. The dji dumb charger and a car battery are both power supplies, the difference is, the dji PS has a 4a limit, where as a car battery has no external limit on the current it can supply, and the last time I tested my RX8 battery, it spat out 1,008 Cranking Amps (cranking amps is short term/high draw rating).