Question: "Does a fast begin and end at the time of Vespers, or at midnight?"
While the liturgical day begins and ends at Vespers, we begin fasting no later than midnight, and we end the fast at some point after midnight, depending on whether or not there is a liturgy at which we will receive communion.
There are some who argue that we can break a fast after Vespers rather than at midnight, but consider what this would mean for Pascha. It is true that at the Vesperal Liturgy on Holy Saturday, we begin to enter into the celebration of Pascha in an anticipatory way (e.g., the liturgical colors are changed from black (or purple) to white at this service); but we do not break the fast. In fact the ecumenical canons specifically forbid breaking the fast before midnight:
Also, except for infants, or those with health issues, we begin a complete fast from all food and drink from the midnight before the day of the Liturgy at which we will receive communion."The faithful celebrating the days of the saving Passion with fasting and prayer and contrition must cease their fast about the middle hours of the night after Great Saturday, the divine Evangelists Matthew and Luke having signaled us the lateness of night, the one by adding the words “at the end of the sabbath” (Matt. 28:1) and the other by saying “very early in the morning” (Luke 24:1)" (Canon 89 of the Quinisext Council).
The Importance of Fasting and its Observance Today (Draft document of the Pan-Orthodox Council, adopted by the 5th Pan-Orthodox Pre-Council Conference in Chambésy on October 10-17, 2015)
On the Participation of the Faithful in the Eucharist (Document approved at the Hierarchal Consultation of the Russian Orthodox Church, February 2–3, 2015 in the Cathedral of Christ the Savior in Moscow)
Living an Orthodox Life: Fasting