What tool would you use to cut through cinder block?
Since concrete blocks are fairly thick, I'd drill several holes along the line you want to cut with a masonry bit, then use a use a mini jack hammer such as a small DeWalt with a 1/2"  chisel bit.  The hole will not be perfect shape etc, so you will need to fill around the vent after installed with some mortar.  The pilot holes will help keep the hole in a rough shape and avoid breaking off large pieces of the block.  Even it some block breaks, you can fix it up after with the mortar mix.