Job cuts among financial and professional services firms will cause office fundamentals to weaken in Boston this year, but modest amounts of new construction will temper the supply and demand imbalance. With layoffs at State Street Bank, Bank of America, Merrill Lynch and Fidelity Investments projected to total in the thousands, a resulting decline in office space demand will drive up vacancy for the second consecutive year. In the CBD, negative net absorption of approximately 550,000 square feet will raise the average vacancy rate nearly 200 basis points to the high-11 percent range. While tenant demand across the metro will wane in the near term, tighter construction financing and lingering economic concerns have reined in development activity. Completions in 2009 will drop off from last year and will represent only a 0.6 percent expansion of metrowide inventory, helping to offset reduced employment-generated demand. Weakening fundamentals and an uncertain economic outlook will underpin conservative buyer expectations this year. As a result, deals will be underwritten assuming higher vacancy rates and rent declines, elevating cap rates metrowide. Currently, initial yields are averaging in the high-6 percent to mid-7 percent range, up about 25 basis points to 50 basis points over the past …