Can a conductor retain charge in vacuum/if it is insulated?
You might want to check out real-charge Electrets.

You can make an electret by stranding an electric charge inside of a dielectric (insulator). You typically do not use a conductor to hold the charge for practical reasons, but I can't think of any reason why you couldn't if you wanted to. 

Depending on how good an insulator you use, the excess charge can persist for decades or longer.