Whatever scheme you choose, use something compatible with the concrete, not plastic. For example, some simple steel wire could be wrapped around the PVC one turn in the middle and cut to the length needed to hold it centered. Might take two pieces for best centering, but it would be quick and easy. The steel and the concrete will work well together, like rebar, you know. HTH
We always use tie wire through holes in the sonotube to hold these types of things in place. Drill some 1/8" holes in the sonotube on opposing sides and slip a double strand of wire through the holes and around the pvc and out the holes on the opposite side. Twist the wire from both sides, pull one side or the other to align, and you are done.
We had a situation once where we staked the bottom of the pvc in place using rebar driven into the ground and tie wired the pvc to that. Then we slipped a piece of galvanized pipe down into the pvc and placed a 2x4 (with an appropriate sized hole drilled in it) over the top of the pvc/hard pipe and secured it in place to hold the pipe centered. Held the entire run perfectly straight.
Unless you are doing something special, it doesn't really need to be centered perfectly all the way down, just at the top and bottom (and not really at the bottom, but that makes it easier to keep plumb).
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