The city of Vallejo this morning declared a state of emergency that limits rent increases to no more than 10%. The declaration says the city manager, acting as director of emergency services, has the authority to make such a move when the City Council is not in session.   The measure, which expires after seven days, is intended as a stopgap until the City Council can vote on its own declaration. The City Council is expected to hold a special meeting next week for that purpose, but the date and time has not been announced.