Your radon stack exhaust termination is in your vented attic space?

Yes, located in unfinished vented attic. And I wonder if will even condense inside the wall where the vertical pipe runs through

Well it shouldnt be terminated inside the attic for one thing. Condensation on outside suggest high humidity in the attic which is peculiar for the wintertime and suggest that there is a lot of air "communication" from the conditioned space below. Not good for Indoor air, energy costs or the radon situation in general. How old is the house and have you ever air-sealed the attic floor/ceiling plane? I would also consider turning down the radon fan if possible.

I have never heard of venting inside. I thought inside venting was against code. Since Radon gas is heavier than oxygen now you have 2 sources entering your house.

Venting inside is definitely against code! Eliminating radon is about dissipating it in the atmosphere and not allowing radon to collect in any enclosed space as concentrating radon can lead to creation of other undesirable radioactive products.
Since radon system is sucking air from under the slab, it follows that it collects moisture from the ground (This leaves a drier and less humid basement as a bonus). This moist air is being dumped in your attic. If this builds up, it can lead to mold problems. Need to extend pipe outside. Exit should not be near an openable window. PVC drainage pipe is cheap, so extending it should not be expensive. If need to go through roof, a proper flashing boot must be used--much messier than if can go through wall(silicone caulk around hole is enough)