Optimizer not picking preferred vendor out of multiple contracts

We need to use outline agreements in APO. We are doing the following:

1. CIFing the outline agreement(s) to APO
2. T-lane(s) created from Vendor to destination location
3. Running Optimizer
4. Purchase requisitions created with suitable source of supply.

Problem:

If there are more than single vendors for a material , there is a preferred vendor that needs to be picked by the system. We set the t-lane priorities acc. to preferred vendor and it works fine with heuristic. But for optimizer, we are not able to determine a method for the system to determine the preferred vendor and pick it during optimizer run.

Could you please share your thoughts or best practices regarding above.

Thanks.