@MauryJ yes, that DC 12v car/cigarette jack on the AC50S should have no problem running that fridge. I’ve used it for my iceco jp50 with no issues, which is very similar in power consumption.
Have you tried plugging your vl42 fridge into your car cigarette plug? Or another SoGen (solar generator)?? This would determine whether or not the problem is in the cig plug of the AC50s, the DC output on the VL42, or maybe even just the cable.
Is your fridge running off the cig plug on the AC50s at all?? Or does it run for awhile then turn off as the battery is dropping??
The AC50s has a regulated DC output so it shouldn’t be tripping the VL42’s battery protection mode, but you could try turning that down to “low” on the VL42 and see if that helps? If switching the mode to low works, that would lead me to believe you have the AC50, rather then the AC50s. The AC50 would be allowing your fridge to run for a while, then would stop as the incoming voltage drops below 9.6v.
As far as the AC vs DC efficiency in the unit, I would stick with using the DC output for the fridge. There will be added losses with changing DC to AC.
On the Facebook group for the “bluetti portable power station club” there was a guy who ran his VL45 and a coffee grinder at the same time with no issues for 44.5hours so the Bluetti AC50s def can handle the fridge, we just gotta determine where your issues are coming from.