I can't comment on your VNA measurements as I have no experience with VNAs. Mine hasn't been delivered yet but I do know about antennas. Your predicament is not going to get solved with VNA graphs & smith charts.
Were I in your situation, I would certainly feed the antenna from the high (altitude) end and keep the other end of the wire as far away from any metal as possible. I would somehow find a ground from either a water pipe assuming it's not plastic, or at least use the earth wire from your AC mains connection. Feed this via an ATU. My old TS-440 has a tuner built in and drives a 40m full wave loop antenna at 20 feet or so above ground. It works on 80m, 40m, 17m & 15m. Let's say it loads up and radiates. My 10Watt WSPR beacon has been heard all over the world with this setup.
I have used some pretty weird antennas in the past when portable. Fences, disused telephone aerial telephone cables, Power pole strainer wires coming out of the ground. Metal rainwater drain pipes, a 40m dipole laying on the roof of heritage steam train carriage while under way. All good fun and educational.
As we say in VK land , suck it and see. Experiment.
73 Bob
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No good deed ever goes unpunished.
Bob