I have been using VNAs at work since the '80s. Professional "box" VNAs have extensive internal power supply and data line decoupling/shielding and are designed as a system with their PC/controllers/display, all housed in a heavy metal EMI-shielded enclosure where all ingresses are carefully filtered, with a chassis grounded via power cord.
The pro USB-based VNAs (Copper Mountain make outstanding units) usually come in a stout shielded housing and generally have a ferrite (or one on each end) molded into the provided heavy and presumably well-shielded USB cable and the user is cautioned to use that cable only.
Quite a different situation from an inexpensive handheld device which communicates with and is powered by a skinny $1cable that doubles as a dandy RF pickup antenna going into a random poorly-grounded or ungrounded laptop or charger with unknown USB power quality. So it is not unreasonable to expect to have to do some RF decoupling work when using these amazing little instruments, especially when measuring antennas.
They may have their issues but just step back and think of what your $50 or $100 buys; they can do 95% of the work that the multi-$100K Keysight ones do.
73, Don N2VGU
Donald S Brant Jr