If one of your rental is vacant for 6 months can you take a detuction for lose of 6 months rent

Your loss would occur at the "gross" level. Are you trying to double up?

It is not a loss. It is just that you do not have rental income for those 6 months. For rental property complete schedule E (Form 1040).

If you have expenses (real estate taxes, insurance, etc.) during the period held for rent, you may be able to deduct a net loss.