We have recently upgrraded from a dd640 to a dd4200 defined as a vtl.
i have 19 access groups (AS400) over 6HBA's.
wiil ddboost work in thie environement
DDBoost is not support by AS400, so only VTL is possible here.
If your DD is licensed for DDBoost, of course you can use it with other DDBoost compatible backup software (e.g. EMC Networker) in parallel with your existing VTL AS400 environment.
Hope that helps.