I’ll bite… First, you can use your multimeter to check the DC conductivity through that adaptor and it should have zero ohms between the two sides of both th case and the centre conductor. Seems unlikely to be open circuit but worth checking.
Then you need to setup a test that uses that adaptor to see whether it is faulty at 146 mhz. The VNA should be able to do that but you’ll need an appropriate load, or the antenna to use as a known good load. If the VNA reports that the load is not a perfect one if that adaptor is in the path to the known good load, the adaptor is under suspicion.
It is hard for an adaptor to be seriously faulty at that relatively low frequency.
The standard way of checking an adaptor is to swap in another just like it and see if there is any difference. Guess you would have done that already if you had the second adaptor. For such testing on air, in the absence of a local signal source (like a signal generator or a communications test set) it’s probably best to use a signal that is reliable but weak. A distant beacon signal if there is one, or a known distant station on the input to a repeater they frequently use. Or a distant repeater that you can hear reliably. The point about the signal being weak is that if it is weak on one adaptor, or one radio, but is unreadable on another, you know there is a difference in either the connectors, the adaptors or the radios. Eventually you have to rule out the connectors as the source of the fault by proving they all work equally well, then you’re left with the radio.
This is why many of us have a really good collection of between-series adaptors and patch cables etc. they are really valuable for testing like this.
Good luck. Would be nice if it’s a $5 adaptor at fault and not the new $300 radio.
73 Andrew VK1DA/VK2UH