Mercury News management is threatening layoffs if it can’t get concessions from unionized workers, the Guild reports in an update to its members. Among the concessions sought is a four-month freeze on the accrual of vacation time and the ability to impose a one-week unpaid furlough. The talk of layoffs comes amid reports of declines in ad revenue in November and the first half of December, the update said. Two part-time sales assistants have already been laid off, the update said.