remember that a surface will only only attain a temperature of that which is heating it... so basically the hottest a basking spot would get is as hot as the rock will get at that particular distance. in a vacuum the rock would heat to the temperature of the bulb's surface temperature at the glass, however having air inbetween the two objects reduces the amount of energy arriving to the rock... SCIENCE!!!! so as long as you are not having it closer than it would normally be for a basking distance you dont have to worry about it heating up too much... it can however effect the ambient air temperature of the vivarium so thats something you would want to watch for.