Hide aquarium heater in gl vase?

I do not think that putting the heater in the vase is such a good idea since the vase itself might prevent the heater from heating the whole tank. Since your turtle is not actually sitting on the heating element I don't think he is in any danger. I would try calling a local pet shop to see what they think. Maybe you can try taking the heater (or at least the model and serial number) in to see what they'll tell you about putting it in the vase.