Solution

The phrase "had vanished" is in the past perfect tense, which indicates an action that occurred before another action in the past. In the sentence, "had vanished" suggests that the vanishing of the quiet stillness happened before the TV set was installed. However, the sentence's context implies that the installation of the TV set caused the quiet stillness to disappear immediately, which doesn't align with the sequence of events suggested by the past perfect tense. Instead, using the simple past tense "vanished" or another appropriate tense would better convey the intended meaning: "As soon as the TV set was installed, the quiet stillness that existed in my aunt's house vanished." This change ensures that the cause (TV set installation) and effect (disappearance of quiet stillness) are presented in a more straightforward and chronological manner, in accordance with tense rules.