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Hello Bill,

Great to see you are having fun with this. The NanoVNA is such a wonderful piece of kit.
As others have said here, you may not have to sweat this too much.
The 1/4 wave whip on most aircraft is usually 1/8" stainless steel rod and will not be as broadband as yours is now. As someone else noted, the radios are probably pretty tolerant anyway.
If you minimise the VSWR around the ATC and VFR frequencies you will actually transmit on, that will suffice. You will still receive stuff (like ATIS) at the extremities of the band OK.
Whilst the fan and bowtie suggestions for broadbanding the antenna are very valid, they do not seem to be a practical option for your vertical stabiliser looking at your photos.
As for the different lengths, that is quite feasible as other components in the aircraft, including the ply skin and the coax cable itself will affect the tuning.
You have the best tool available in the NanoVNA to do this tuning. Trust your VNA.
Most LAME electricians who install radios in aircraft would not know what a VNA was, let alone how to use it.

Just a word of warning, don't be tempted to connect the NanoVNA to the antenna cable at the airport or around other aircraft who might transmit near you.
Also, don't be tempted to use a hand held radio at home with the VNA connected.
You run the very real risk of frying the NanoVNA CH0 input if you do.

Good luck with your project. Hope you get many hours of enjoyment flying it. And many more hours of fun playing with the NanoVNA.

Cheers...Bob VK2ZRE