The Vetted Relay list is composed of 2 sub-sets:
An NO / Cluster (in DVT) must use at least 1 from must use some, ideally as many as possible (or operationally feasible, as some relays have bad performance in certain geos). NOs / Clusters don't HAVE to use ALL relays, but each cluster must use the same sub-set of relays to minimize issues. Clusters who are "filtering" clusters due to the requirements of some of the participants should similarly use a shared sub-set of (or) all filtering relays.
Lido on Ethereum: Call for Relay Providers
Lido on Ethereum: Identify and constitute Relay Maintenance Committee
Relay Maintenance Committee - Feb 2023 Proposal
RMC Proposal - Remove deprecated Eden relay, Add Titan relays