1. Question: If a person took upon himself a personal fast right after shabbos, that night, why can he say havdala over wine from plag hamincha and drink the wine but if one would make havdala from that time when Tisha b’av falls out Motzei shabbos/Sunday then he can’t drink the wine because he took on the fast?
A. A personal fast is a vow and a vow is based on the language of how it was said. And how something is said is based on how people normally say things.
B. The acceptance of the fast of Tisha b’av is dependent on havdala if it falls out on Motzei shabbos.
Answer: When a person says he’s taking on a fast after shabbos he does not considered the, while it is still light, as an acceptance of the fast if he makes havdala for he means to start the fast at actual night but on Tisha b’av as soon as havdala is made the fast starts, so it’s forbidden to drink, even if havdala was made at plag hamincha. (See Dirshu Mishna Berura 556:1:2:3)