Linear time, wau.
When i am building that hash and storing what i have seen so far, it's getting bigger and bigger and i have to search it to find a vertex if it's there or not, it doesn't look like linear. Perhaps i have to think about it more it will be nice to see some pseudocode.
edit: i just saw some small examples using hashes and i have a better view now
edit2: there can be another problem, most of verticles don't have to be exact same, but there can be small difference 0.001, so these will be "warped" into one.