"Voltage transfer function" by itself is not completely
meaningful at high frequencies.

One classic
definition is Vout/Vin, when driven from a voltage
source, and driving into an open circuit. This can
often be approximated at low frequencies (audio),
but even there, you must be careful not to make
your measurements inaccurate.

Both a pure
voltage source and an infinite impedance load are,
at some point, impossible to obtain and still be
useful. For example, an active oscilloscope probe
with 0.5 pF input capacitance still has an input
impedance in the hundreds of Ohms at 1 GHz.

If you're using a VNA, you have a 50 Ohm source
and a 50 Ohm load. You may be able to compute the
voltage transfer function mathematically - but I'm
not sure which constraints are important to you.
The information is "hiding" mostly in S21, but needs
compensation for S11 and S22, probably. Our host
here at RFCafe has posted a complete set of equations
for various conversions to and from S-parameters
at

http://rfcafe.com/references/electrical/s-h-y-z.htm
You'll need to identify what you really need,
either as a Z-parameter, H-parameter, or Y-parameter.

Good luck!