How to activate Dual Control ?

Hai Group

My clent wants to activate Dual control for vendor or customer.

we have made some fields as Sensitive in vendor maste.

even after this when any user makes changes in the vendor master the system is saving the changes and even it is allowing to post entry for that vendor without blocking it.

please suggest us how to prevent user to save the changes in master data of vendor ( it should get approval from higer level), and how to block posting for the same.

Thanks and regards

Karthik.