Yeah, that type of telepathy wasn't really introduced until later content. Also, you could argue Leia's diplomatic training coupled with her innate Skywalker-level connection to the Force protected her from that kind of mental probe.
Retroactively speaking, Vader definitely knew the ability... Even back in TCW, he(along with Kenobi and Mace) tried to TP-rip the info out of Cad Bane's mind.
Ben Solo in The Rise of Kylo Ren mentions that the ability to rip information from the mind of a target is just a variation of the typical Jedi Mind Trick. Granted, a very dark application of the Jedi Mind Trick, given how we see the recipients respond, but a Jedi Mind Trick nonetheless. I highly doubt Vader didn't know how to do the ability by the time of ANH.