I just read the short story about Adalric Brandl, a former Jedi Knight who was serving the Emperor, age unknown, but likely in his 30's - Vader's age at this time, too.
I think that either Vader didn't know, and of course Palpatine kept his secrets, or these were Jedi who could be corrupted as Dooku twisted some (including Quinlan Vos very nearly), or they were blackmailed like Ferus Olin, (approximatley 25 years old during The Last of the Jedi as Vader would be 22-23 and Ferus was slightly older than him) or they were taken as infants like Mara Jade.
I wonder if Palpatine knew about Brandl's kid: Jaalib Brandl - a likely Force-sensitive, hidden on Trulalis? Was his safety Brandl's price? Would Palpatine honor their bargain after Brandl's suicide?
Palpatine was into manipulating everyone and everything. No doubt he got Force-talented Emperor's Hands by every dirty maneuver he could possibly make. I wonder if he'll try to replace Vader during his 19 year reign before their discovery of Luke following Yavin?