I bought Vhailor's Helm and put it on Sir Anomen. That gives him Simulacrum - as a "Use Item, Vhailor's Helm"
But I see no effect at all - isn't Simulacrum supposed to create a weaker copy of the original?

have you tried activating it through the items option at the bottom of the screen (the little gold coloured bag)

Yes, I do that and the Vhailors Helm/Simulacrum symbol is visible with a "1" for 1 charge.
When I click on that icon, I just have gotten "Sir Anomen, Save vs Death" and the charge
is used up.
However, I have tried this many times during my response to your question - and finally the
Simulacrum is created. And the "Save vs Death" text isn't displayed.
So, it seems that only if the person FAILS a "Save vs Death" the Simulacrum succeeds.
And since Sir Anomen has that saving throw a "3 (-1)" a d20 won't get a simulacrum created
very often...

Hmm, try it on someone else. I have no problem with this helm on my PC or on Korgan, and it is very powerful (though often dispelled by liches).