Is the reason neither Yoda or Obi-Wan thought it a good idea to inform Luke that his father and Vader were the same person ever discussed? Were they hoping that Vader himself didn't realize this, and that it wouldn't come up? Or was there an explicit rationale behind hiding this fact from Luke?

Wouldn't it seem risky to send Luke in blind, with the chance of him finding out without their support, considering how emotional attachment can be a door into the dark side?