Funny enough the answer is right in the definition of
Critical Hit in the Archives in the Game Terms section. This is followed by Damage and then damage.
The key to identifying damage is in the letters. If something grants damage like +10 damage because it is a lower case d that is how you know it it added later and does not change the base damage on the card. In your example the damage is a lower case d therefore Sith Rage is not included in critical so Vader crits you have +20 base then +10 for rage and finally +20 for multiplied critical hit for a total of +50 damage. Some cards, like the Vong Shaper and Genoharadan Assasin, have situations you can meet to amplify their base damage because they say +10 Damage with a capital D and can thus increase the critical hit damage.