To do a septic tank cleaning you must open the tank, have means to get the waste out, and have a place to put it. This can usually be done with a septic truck which has a tube which will suction most of the waste from the tank.
It is important to have your septic tank cleaned regularly and properly. If you don't you could have a major (gross) problem on your hands. Ask your neighbors which service they use to have their...
There is no national cleaning agency for septic tanks. The best resource to find a septic tank cleaner is a local yellow pages or the company that installed the original.
The "septic system vent" you are referring to is not actually attached to the "tank" (that would be the buried, holding chamber in your yard) at all. It is attached to the plumbing in the residence...