for a Frenzy or Berserk barb: Is Silence a viable weapon to use at all, or is it just plain bad?

Let's say even if you discounted the fact that it requires a Vex and just evaluate it against other midrune weapons. The ed% is low and the 25% monster flee would probably get annoying fast. The only thing it has going for it is the skills and nice resists. Should I just completely overlook this weapon?

Yes, disregard it completely. ED% is too low. No CB either. +res can be found in other items, charms and even natural res. +skills too, items, charms and BC.

And that monster flee will get annoying fast.

It costs almost the same as Grief so you should not have to think twice