user: Question: What is the date of vacancy when the team is Manchester City and replaced by is Mark Hughes?
Columns: ['Team', 'Outgoing manager', 'Manner of departure', 'Date of vacancy', 'Replaced by', 'Date of appointment', 'Position in table']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Manchester City', 'Stuart Pearce', 'Contract terminated', '14 May 2007', 'Sven-Göran Eriksson', '6 July 2007', 'Pre-season']
SQL:

assistant: SELECT "Date of vacancy" FROM "Table" WHERE "Team" = 'Manchester City' AND "Replaced by" = 'Mark Hughes';