11 Glaring Plot Holes in Star Wars

2. Becoming a force ghost requires training

The original trilogy, all you had to do to be a force ghost was just to die serving the Light Side of the Force, which is why Vader can come after performing a single act of good despite 20 years of serving the dark side. At the Revenge of the Sith, Yoda implies that this isn’t the case:

Yoda – “An old friend has learned the path to immortality. One who has returned from the netherworld of the force, your old master.”

Obi-Wan Kenobi-“Qui-gon?”
Yoda-“How to commune with him, I will teach you.”

Both Yoda and Obi-Wan have to study and train with the ghost of Qui-Gon to become immortal. Yoda even has to teach Obi-Wan how to communicate with the ghost of his master.  And given how Vader wasn’t in the good books, it’s unlikely he would have picked up on the secret in passing.