This is a bit ambiguous, but let's proceed by considering that any capitalized word that's part of the main title or heading is a proper noun. Therefore, "The Impossible Vietsub" is a proper noun and stays. Then, in the sentence "The Origins of The Impossible Vietsub", "The Origins" is part of the subheading. If that's the case, those "The" and "Origins" are part of the proper noun title and should stay. Alternatively, maybe "The Origins" is not a proper noun. This requires careful reading.
Given the ambiguity, perhaps the safest approach is to treat only "Vietsub" as the proper noun and replace all other words. However, the user's example might indicate that the entire title is kept. Alternatively, maybe the user wants all proper nouns (names, places, etc.) to stay, while the rest are replaced. Since "Vietsub" is a term here, if it's a specific entity, it's a proper noun; otherwise, it's not. Since the user wrote "Vietsub" as a proper noun, it's part of the title. Then "Unraveling the Mystery" is part of the title but not a proper noun, so the words in it should be replaced. the impossible vietsub
The first sentence is: "In the vast expanse of the internet, there exist numerous mysteries that continue to baffle and intrigue users." This is a bit ambiguous, but let's proceed