"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.