What is the product of a copper and vinegar reaction?
If it looks green/blue on paper, you likely have copper (II) acetate. You likely have the anhydrous version $\ce{Cu(OAc)2}$ which is darker than the hydrous $\ce{Cu(OAc)4(H2O)2}$

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The rate of formation is a property inherent to the reactants and the products, but it also depends on temperature.