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David,

Yes, you can use the VNA in the shack calibrated in the usual way
with short cables. It will see what the transmitter sees.

Calibrating with your long run of coax instead of the short cables
allows the VNA to mathematically remove the effect of the coax to the antenna.
The swings in SWR between resonance and non-resonance will be
significantly larger, because it subtracts out the signal attenuation
that the coax introduces.

If the coax were perfect and did not attenuate your signal, the SWR would be exactly
the same by either measurement method. But the combination of resistance and
inductive/capacitive reactance to create that SWR will be very different for different lengths of the coax.
The VNA can calibrate this out as well, telling you if the antenna is capacitive or inductive,
That is of interest in some cases as it can tell you exactly how to adjust things up there
to get better results. But that is not of much interest to most hams, they just use the SWR
to figure out how well their antenna is doing.

No, you're not dumb.
This stuff gets complicated fast.
Figuring out how to use a VNA to measure SWR can be done in a few minutes,
assuming the instructions are easy to follow.
Figuring out everything the VNA can tell you about an antenna system
and how to make good use of that information could take years.
Just fIguring out the VNA is an excellent hobby.

Your EFHW-4010 is a good choice, easy to set up without need for
running coax up into the middle of the wire, does a good job of hitting multiple bands.
As of 3 years ago, Danny told me that the EFHW-8010 was different only in the
length of wire, so you might be able to add 80 meters to your antenna by simply
adding wire to the end if the opportunity ever comes up. But performance will
vary a bit depending on the wire gauge, copper vs steel, stranded vs not, type of covering.
I found I got significantly different results using some stranded copper antenna wire
I had lying about here than I did with his wire. A good reason to have a nanoVNA around.

Jerry, KE7ER