Veileds to my knowledge can experience temps as low as 60 without an adverse affects. Possibly lower, however I would not go below 65 at night, but I'm sure your cham will be fine.
As for the basking lamp, I don't think it is necessary for outside, I could be wrong. As long as you have some shade and cover in the cage, and an area where the cham may sit uncovered to bask in the sun, it should be fine. I also beleive you should have the cage half in and half out of sunlight if possible.
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