When does God do the forming?
When the clay is on the wheel being fashioned into what the potter wants it to be, not when he's creating the clay from nothing.
Pharaoh was 'made' a vessel destined for destruction by God
when he hardened his heart against the Lord. That was God's prerogative to do so. And He did that based on Pharaoh hardening his heart, and in His foreknowledge that he was going to do that. And that decision to reshape the clay and harden it into place can not be challenged or questioned, even by the clay itself--"Why did you harden me? You know I can't resist your will in this matter?" No, you can't Pharaoh. The Potter decides what he's going to do with the clay that can not be molded in his hands.
I apologize for the lengthy quote, but Jeremiah 18 helps us to see that explanation is more in line with what Paul is getting at:
"1The word which came to Jeremiah from the LORD saying, 2“Arise and go down to the potter’s house, and there I will announce My words to you.” 3Then I went down to the potter’s house, and there he was, making something on the wheel. 4But the vessel that he was making of clay was spoiled in the hand of the potter; so he remade it into another vessel, as it pleased the potter to make.
5Then the word of the LORD came to me saying, 6“Can I not, O house of Israel, deal with you as this potter does?” declares the LORD. “Behold, like the clay in the potter’s hand, so are you in My hand, O house of Israel. 7“At one moment I might speak concerning a nation or concerning a kingdom to uproot, to pull down, or to destroy it; 8if that nation against which I have spoken turns from its evil, I will relent concerning the calamity I planned to bring on it. 9“Or at another moment I might speak concerning a nation or concerning a kingdom to build up or to plant it; 10if it does evil in My sight by not obeying My voice, then I will think better of the good with which I had promised to bless it. 11“So now then, speak to the men of Judah and against the inhabitants of Jerusalem saying, ‘Thus says the LORD, “Behold, I am fashioning calamity against you and devising a plan against you. Oh turn back, each of you from his evil way, and reform your ways and your deeds.”’ 12“But they will say, ‘It’s hopeless! For we are going to follow our own plans, and each of us will act according to the stubbornness of his evil heart.’" - Jeremiah 18:1-12
It isn't that the Potter purposely
created the clay to be uncooperative and, therefore, destined to be molded and hardened into a vessel set apart for destruction. But rather the clay was uncooperative when molded and so the Potter, entirely at his own discretion, fashioned and hardened it in place into a vessel set apart for destruction, not for good. And He did that to show the glory of Him not doing that to other pieces of clay.