I seriously believe nobody really knows. I have read on reddit that they proc around 10-20% of the times you are hit. No serious data tho.
"Truth is like a lion. Don't try to defend it. Let it loose. It will defend itself" Augustine of Hippo
With the Invoker's Set being buffed in patch 2.4, Sanguinary Vambraces seem to me as an interesting item for the build or even maybe BIS?!
The problem is that I do not know how often they proc. Has anyone actually tested them? No speculations please...