He seems to be asking why you need the bridge to measure S21. I took it that you feel you can make these measurements with S11, possible not understanding the errors that go along with it for this particular application.
I was only responding to "my VNA won't go to 10 Hz and I can't afford a 4395A".
I got super lucky as shown by the screen shot. The seller didn't know what it was and wanted it gone. They were in the business computer business. So far as I can tell it works flawlessly. And I've used it a lot as a scalar network analyzer evaluating combinations of EMI filters.

VNAs have directional bridges in them. I was simply pointing out that you don't need a VNA. A signal generator, DSO and understanding the problem will solve it.
To the frequency limits of the DSO, analog log amplifier probes and some software are capable of providing full VNA capability from DC to the limit of the DSO you currently have. Any that will write data to an SD card will do.
I bought a cheap 12 bit 100 MHz Owon because it has very deep memory, 20 MPt IIRC. Sucks as a scope but is great for data capture. A perfect square wave with fast rise times will generate harmonics of very high order. Choose the fundamental based on the sample interval in frequency and use a sub ns rise time 7400 series divide by two output. Record it open circuit and then measure at various points. The processing understanding is a one time effort and valuable skill to boot.
That said, my favorite tool for impedance discontinuity tests is a Tek 11801 and SD-24, but that is rather exotic and hard to come by. But it does show the precise (<0.1 mm) relative location of *every* impedance change. Just not well suited to a use case where an active device is at the other end of the line.
As should be clear from the various links, this is NOT a simple problem to solve even with a large sack of money to apply as a hammer.
After rereading the OP a few times I should start with a DSO and a very small DIY H field probe to isolate which segments of an actual PDN have issues as a first step. Small loop of the smallest coax you can find. Preferably semi-rigid. You'll be able to directly detect what parts of the PCB have problems.
Then use an FFT to get a frequency domain view to give a sense of scale to the parasitics of concern. Build the fixtures in the Keysight App note and measure them.
A differential DC log amp probe with a high input impedance should overcome the DSO dynamic range limitation. In principle, one should be able to probe two nearby points on a trace to measure the current flow changes. We all know that there is a voltage change as the current varies, but whether that's actually practical as a measurement technique for this use case is another story. I suspect that careful construction would make it viable as a punch through the solder mask probe. But someone will need to try that out to know for sure. Almost anything else has ground loop issues to be addressed.
Have Fun!
Reg