Are you sure it was the external mic that was picking up the sound and not the built in?

Yep, pretty sure. The internal mic gets deactivated when something is plugged in.

I just used a DVX100 today, the audio set up is the same.

2 XLR inputs, audio level dials for each channels and two switches for channel one/two

either:

Internal (L)
Input One
Input Two

for channel one

or

Internal (R)
Input Two

For channel two

Wow, the VX2100 manual is abysmal. I never noticed before how poor the coverage of using an external mic is.

But the VX2100's external mic jack is a stereo jack in either mic or line mode. So Aviv can use a Beachtek adapter to apply one mic to both left and right channels, or to apply one mic to the left and a second mic to the right. Either of those things can also be done with cables and plug adapters, but units like the Beachtek have advantages.