VSS (Volume Shadow Copy Service) is the underlying architecture that MSFT implemented in Server 03/XP for backups. It's a pretty well documented system with APIs that can be called in order to do backups. VSS in and of itself is not the backup solution itself, its just the service. NTBackup utilizes VSS to take it's backups, but it's pretty limited.
DPM is MSFTs entry into the server backup and recovery market. It utilizes VSS to take point in time snapshots of data and stores them on disk, which can then be offloaded onto tape or other media for long-term storage.
Whether or not you should utilize DPM in your environment is up to you. It comes down to what works best for you and your environment and budget. Some people like the price point, but think that managing the system is too much of a headache. Other people love it.
The best advice I could offer is to download their trial version and play with it for a while.