The participle 'prepared' is difficult. It is not as easy in Greek as in English translations to simply parallel the vessels of wrath prepared for destruction with v. 23, the vessels of mercy prepared for glory. This is because the participle in v. 23 is an active participle, much more obviously being connected with divine preparation; whereas for prepared in v. 22, Paul uses passive participle in middle voice which does not immediately suggest God as the one who prepares.
So what do we do with this? The answer I believe again is deducible from the context. Paul seems to be drawing an obvious parallel between vv. 22-23 and vv. 17-18 (see my previous post on Rom. 9 'the participle thelōn' for more on this), and here is no exception. God is the agent of Pharaoh's hardened heart, and is further the agent in Pharaoh being raised up, God is there, the one who prepares. If this parallel stands, and due to the theological and contextual flow of Romans 9, I believe it does. Then it is indeed God who prepares the vessels of wrath for destruction...for the purpose of showing his wrath, his power, and his mercy, ultimately, for the purpose of magnifying His glory.
'He does not indeed give a reason for divine election, so as to assign a cause why this man is chosen and that man rejected; for it was not meet that the tilings contained in the secret counsel of God should be subjected to the judgment of men; and, besides, this mystery is inexplicable. He therefore keeps us from curiously examining those things which exceed human comprehension. He yet shows, that as far as God's predestination manifests itself, it appears perfectly just.'
[J. Calvin on Rom. 9:22]