Raster graphics are much larger files since they store data by the pixel. For a black and white image it stores the gray scale value of each pixel and if you are operating in the proper range you should be getting 2-3 pulses per pixel. This is perfect for photos and the laser can create the image either in true gray scale which is hard to get dialed in or with dithering which is much easier but does not produce as good of quality engravings.
Vector graphics converts everything to geometric entities like lines, arcs, curves, etc… much less data and if you are lasing only in 1 or 2 colors is much less taxing on the software and hardware since there are not near as many commands being sent.
Raster graphics are dealing with huge amounts of data very fast. If you are not engraving images, best to stick with vector data. If you are engraving photos and want decent quality, try to stick to no more than 300DPI and 50 to 100mm/sec. this is just my experience as of lately. It also depends on the quality of the laser tube and power supply.
Since you are engraving monotone, I would think that vector graphics would be the way to go.
Anyone reading this, please let me know if I have not quite explained this correctly or can add to or correct my interpretation.
Just food for thought