At the following link
"FieldFox offers the key functionalities of the HP 8058A, in a handheld form-factor, and without the need for the bridge and accessories required with the HP 8058A.."
I find that a bit hard to believe, as the HP 8058A is a sound level meter! Was an HP 8508A vector voltmeter meant instead?
During my undergraduate project, some 30 years ago, I used an HP vector voltmeter - I forget which model. That was a high input impedance device. It would be worth pointing out that this option 308 on a FieldFox is a 50 Ohm input impedance, as many people will tend to associate voltmeters with high input impedance. I know I thought this, when I bought a FieldFox.
To be honest, I felt the vector voltmeter option was a pretty useless option if you have the S-parameters, as it tells you nothing you can't tell easily from another graph. The only advantage it seems to have is the numbers are big and easy to read. It does not exactly provide a lot of functionality, given its cost.