Payment Delegation (PDG) - This is a group for signing transactions by selected participants from initial members or their representers.
Vesting - it is an economic model for maintaining initial validators. Tokens are locked and issued monthly via giver. (See more - freeton.org/dod, Addendums)
The payment process is initiated by the initial members (multisig givers, or MSGs), with a minimum quorum (50% + 1) required for a transaction. This often takes some time in order to get everyone together to vote. This will be fixed in the future. Once confirmed, givers perform transactions to the PDG, which consist of 5 of the initial members voted to transfer payments to the smart contract by the initial members, since it is easier to get a 50%+1 quorum out of 5 members instead of 23. Then everything is transferred to the smart contract for final vesting distribution. This happens every month for 12 months.
Validators tokens are locked until decentralization. All it means is they cannot use them, trade them, do anything with them until after decentralization. Once decentralization occurs they can do whatever they want with them
Vesting is an amount of paid overtime. So take all the tokens that are designated for initial validators and divide them into 12 months.
Here are tables of previous Vesting Payments for Validators: