CREATE TABLE table_16191 ("Team" TEXT, "Outgoing manager" TEXT, "Manner of departure" TEXT, "Date of vacancy" TEXT, "Replaced by" TEXT, "Date of appointment" TEXT, "Position in table" TEXT)

What is the date of vacancy when the team is manchester city and replaced by is mark hughes?
SELECT "Date of vacancy" FROM table_16191 WHERE "Team" = 'Manchester City' AND "Replaced by" = 'Mark Hughes'
SELECT "Date of vacancy" FROM "table_16191" WHERE "Replaced by" = 'Mark Hughes' AND "Team" = 'Manchester City'