Question:

How does a user determine when an Email address was added to the Suppression List in Vault? 

Answer:

An Email address is added to the Suppression List, and therefore delivery of Emails is blocked, when the Suppressed Email Address record is created. The creation date for a Suppressed Email Address record can be found in the Object Record Audit History.

The Suppression Date column in the Suppression List represents when the Email initially bounced. This field does not indicate when the Email Address was added to the Suppression List. 

Related Documentation:

Vault Help Documentation: About the Email Suppression List