Reactivity of KOH with PVC
If you're just curious for practical purposes (i.e., "Can I put this KOH solution in this PVC container or will it ruin it?"), there are many material compatibility tables for this type of thing. This one suggests that your solution will be OK at low temperatures for a few days at least. It may react after more time or at higher temperatures.

If you want to know specifically about the chemistry, KOH is a strong base. Strong bases are known to crack polymer chains through solvolysis. PVC in particular loses some chlorine atoms to the solvent which weakens and ultimately shortens the polymer chains, as explained in more detail in the link.