In general they put money in separate buckets based on where it came from. So in your example $1,000 was contributed through the "employee contribution" bucket and $500 was contributed through the "employer match" bucket. Assuming both buckets were invested in the same thing, the value went down to $500 in the employee contribution bucket and $250 in the employer match bucket. If you leave and are only 50% vested, they'll probably take away $125, but you should check your plan documentation to be sure.