It expects office vacancy rates to hit a high of 16.5 per cent in 2010 - up from an islandwide vacancy of about 2 per cent currently - as firms' expansion plans are hit by the global financial turbulence.Office rentals are also predicted to peak earlier than expected this year, and fall 50 per cent by 2011, said research analyst Shirley Wong, who has downgraded the Singapore office trusts sector to underweight.After her report was released, office trusts CapitaCommercial Trust (CCT) and Suntec Reit saw large drops in their unit prices that put them among the worst-performing property stocks yesterday. Property counters fell across the board as the wider stock market faltered.CCT fell 17 cents to its lowest level in almost four years, while Wing Tai and Keppel Land each dropped more than 6 per cent to three-year lows.