It’s the busiest time of year for rentals in Boston and, unfortunately for apartment-seekers, it’s also the hottest rental market in almost a decade. With consumers worried about purchasing real estate in a down economy, the rental market is heating up with vacancy rates at their lowest level since 2002, according to recent statistics and Boston brokers. The rental market is very competitive this fall with some rents 20 percent more than last year, according to Briggs Johnson, a broker with Coldwell Banker Residential Brokerage in the South End.