NIBE hot water cylinders and buffer vessels follow the labelling format of
Product type - Volume
For example, a NIBE VPB 300 is a VPB hot water cylinder with a 300 litre capacity.
NIBE VPB is a type of hot water cylinder. The VPB serves its function as a hot water cylinder and does not have any other functionality. It's best to let your installer decide if the VPB is the best hot water cylinder for you.
NIBE HA-WH5, more commonly called the Megacoil, is a type of hot water cylinder. The megacoil is installed in a slightly different way than the VPB, but still serves the same purpose. It's best to let your installer decide what kind of hot water cylinder you require.
NIBE UKV is a buffer vessel that forms part of the hot water system. Buffer vessels are specific to each install - some houses will benefit from a buffer vessel while some will have no need at all. It's best to speak to your installer to determine if your setup will need a buffer vessel.