A breeze-block is a lightweight building block made from cinders and concrete.
Breezeblocks, also known as concrete masonry units (CMUs), can absorb some water due to their porous nature. However, the extent of water absorption depends on the specific type of breezeblock and its manufacturing properties. To reduce water absorption, breezeblocks can be treated with sealants or coatings. In general, while they can absorb moisture, they are not as absorbent as other materials like wood or certain types of brick.
Well, technically anything that can breath can drown, but as a vampire is dead and therefore does not need to breathe, they wouldn't logically be able to breathe - as far as I am aware. ;) However, in various folklore stories, people talk of throwing vampires into rivers - this would, as far as I can figure - only work if they were tied to a breezeblock or something. Then again, if you factor in a vampire's unnatural strength... In the original books a Vampire could be killed if it were submerged in the pure running waters of a stream.