The similarity between Vit C & Vit K is
Both vitamin C and vitamin K are required for post-translational modification.
Vitamin C is required for post-translational modification of proline and lysine in collagen synthesis.
Vit K is required for post-translational modification of glutamate residue for generation of clotting factors.
Both help in conversion of proline to hydroxyproline
Both help in post-translational modification
Both have anti infective activity
Both are involved in coagulation cascade