I suspect -- but don't know -- that vacation homes count as "vacant".
The darker the red, the higher the value. Anything less than 20% shows as full white on this map; full red means 100%.
I don't draw borders on the tracts that only take up a few pixels. That's why urban regions tend to have a ring around them with no borders.NOTE: when you are looking at census data, the farthest you can zoom out is to see roughly 2/3 of the US at once. (At some point, I'll make the tiles for farther-zoomed out, but not right away.) In the other direction, Google added a few more zoom levels in API v.2 that my tile server doesn't know about. It usually isn't interesting to zoom in that far, so fixing that isn't a priority for me. You will have a better experience if you clear your browser's cache before doing much.