Don't remove the vent pipes. You can lower (cut) them to within 12 inches of the ground, but don't make them flush with the ground. It is necessary for the system to breathe, and if you have them flush with the ground, they will plug up over time. One other thing, I would avoid planting shrubs, trees, etc over the septic field. The roots will grow down to the tile and plug the system up. Check with your county sanitary inspector, he'll be glad to offer his advice relative to code. Also, leave access to have a truck get back to your septic tank so it can be pumped out every 2 - 3 years. Also, (sorry for running on) most systems have a designated location for a backup field if the first one should fail over time. You may want to check where that location is for your yard, and make sure you don't make it inaccessable.