It is a definite possibility about Vader reading Bail's thoughts. However, it may have been a single encounter with Bail and with as many people in the Senate as there are, I wonder if Vader would have been cognizant enough to know to read Bail's thoughts. Having Leia and Vader together isn't that big a deal. They may not have even interacted at all. And Bail can't stop her from going without raising warning flags in Leia's mind as a result.