Additionally,
If set on a COM / DCOM model then you could also consider the IVI-COM / IVI-C driver for the VNAs.
http://www.home.agilent.com/agilent/editorial.jspx?cc=US&lc=eng&ckey=1669213&nid=-33831.912674&id=1669213.
This is not true COM/DCOM. The IVI-COM driver will encapsulate equivalent SCPI commands as function calls that theorethically should work on many network analzyer families. Note that currently there is no Network Analzyer Classs Compliant IVI-COM / IVI-C driver formal definition so this is a best effort.
Also, your analyzer is an E5061B, if you have options 3L5 and 005 (impedance mode) and you plan to use the Impedance measurement functions I would expect none of this Impedance functionality to be integrated in the IVI-COM / IVI-C driver.
Personally I am not a fan of these drivers (for various reasons left unstated!).
Sincerely,
tsilchia.