In general, septic tanks and fields are not vented. Where exactly did this pipe connect, and are you sure of all its functions? If it was part of an output tee, having that fall into the tank would be bad as that acts as a baffle on the output preventing floating scum from getting out to your drainfield.
Your house plumbing should vent your septic tank unless the tank has sunk and water is completely blocking the pipe. Fields can have inspection ports, but the earth must be porous enough that it doesn't require venting.
If you know nothing about these systems, you should call someone who does unless you want to replace it in a few years.