Virginia state lawmakers are expected Thursday to approve a measure that would retroactively lower the states contribution rate to the Virginia Retirement System to 4.24% from July 2000. The move would cut the states contribution by about $200 million for the current fiscal year, which ends June 30. The state now contributes 5.22% of pay for state employees, and 7.54% for public school teachers. The lower contribution rate, approved by trustees of the $40 billion Richmond-based system in December, was scheduled to kick in for the fiscal year that begins July 1. Trustees condemned the proposed effect on the contribution for the current fiscal year in a Friday letter to lawmakers, but stopped short of threatening legal action.