There are a few different parasites that can cause, as you so delicately put, your itchy ass...:). The most common being pinworms.
A suggestion would be to use some scotch tape, I use the big stuff that is made for packing, and catch some of the BUGgers. Bluepetunia suggests using alcohol, preferably Vodka on a tissue to catch and kill them, then put them on tape to take to a lab to identify. If you get a magnifier you may get a better look at them and be able to identify them yourself.
As far as how long it will take to get rid of pinworms with Vermox....well....there's just NO way of knowing for sure. I took it twice a day for 12 days, off for 7, on for 7 and it had no effect what so ever. But, I found out after that eating a fatty meal, 40gm will improve the efficacy many times over.
Because I have a mixed parasite infection, I decided to try Ivermectin and have been on it for 5 days. The pins are still keeping very busy and so I will continue with the Ivermectin for a few more days, then follow up with Vermox for a week at least.
Bluepetunia is taking Albendazole for pins and so far, seems to be having about the same results as me.
Good luck and God Bless!....YOU ...not the little BUGgers!