Soldered wire or test connector for VNA measurement?
I think your over thinking this.

Outside of super narrow beam pattern horns or the like, 700Mhz is fairly forgiving.

I don't see how a u.fl connector would have enough effect to matter.  As long as your test cable is kept away from the plane of radiation, I don't see a problem.  As for the connector, at that freq a 1/2 wave is 20cm(if your using 1/4 you still need a ground plane), and a u.fl is ~3.5mm x 3.5mm x 2.5mm, see what I mean, to small to matter?  And it should not be in the signal field anyway.  It's not like it a N connector or something like that...hihi

As far as calibration goes, I would just make a little board with three connectors on it, one shorted, one open and one with a 50 ohm resistor across it.  Then I would put right(secured somehow, maybe a tie wrap?) beside the u.fl that I'll be using to connect.  As long as it's close, it should not be problem.

As for what matters?  What is the application?  Is it mounted or mobile?  Is the antenna an omni, or directional?  I mean if your antenna is a coiled 1/2 wave, there is not much that will be critical as long as your transceiver is happy.  And if your receive only, then there is nothing is critical, just helpful.