Q: When I go to the Codex my Vor's Prize quest is blurred out and can't unlock/complete it.
A: When the Quest system was first introduced on PC on July of 2014, users were given the option to 'Skip the Tutorial' for them to continue playing the game without having to play through Vor's Prize Tutorial Quest.
The option to skip this quest has been long abandoned, so new players and players returning from pre-quest system update can play through Vor's Prize without issue and will not experience the above problem.
However, for those who has decided to skip the quest, whether intentionally or not, the Quest cannot be unlocked nor played.
Fortunately, skipping Vor's Prize still marks the quest as 'Completed' on server side, and will not hinder unlocking quests that requires the completion of Vor's Prize (ie. Saya's Vigil).
Warframe Support is not able to change your account status to make the quest playable, nor can we edit your Codex to shows the quest as completed.