vread() accesses the virtual array element. The prototype is void "*vread(hdr *p, long index)". The hdr pointer, p, is the virtual array. The function thus may be thought of as reading p[index]. The block number for p[index] is calculated. If that block is not in the buffer, and the buffer is dirty, the buffer is written back to disk before the requested block is read.
vwrite() writes data out to $$$$vmem.tmp at the indicated hdr and index.
Neither of these functions should be called in a YAMP program since these functions are encapsulated in the virtual vectors and matrices.