Phenolphthalein in Soap Solution
Phenolphthalein is a pH indicator that is colorless in acidic solutions and pink in basic solutions. When added to a soap solution, which is typically basic due to the presence of alkalis like sodium hydroxide, phenolphthalein will turn pink.

Reaction with Vinegar
When vinegar, which is acidic due to the presence of acetic acid, is added to the soap solution containing phenolphthalein, a neutralization reaction occurs. The alkali in the soap solution reacts with the acid in vinegar to form water and a salt. This reaction causes the solution to become neutral.

Color Change
As a result of the neutralization reaction, the pink color of phenolphthalein in the soap solution will disappear. The solution will turn back to its original colorless state, indicating that the pH of the solution is now neutral.
In summary, when phenolphthalein is added to a soap solution, it turns pink due to the basic nature of the solution. When vinegar is then added to the pink solution, a neutralization reaction occurs, leading to the disappearance of the pink color and the solution returning to its colorless state.