Technomagos, I've heard that grape plants will send roots as far and deep as necessary to acquire the nutrients they need. It's also said one grape plant can be cultivated and trained to cover an entire acre
- they're vigorous dudes!
I've have two main septic drainfield lines crossing within several feet of some very vigerous grape plants, and for a dozen years now have had no problem. The extra-green
patches of grass following these lines well beyond the grapes show me they've not been plugged.
Having dug out established grape plants I've noticed their roots (like those of most plants) hang out in the richest soil available, generally up high. As noted, they can really dig deep - like they do in the poor hillside clay soils here in Yamhill County. But if you've got good topsoil I doubt they'll wander too far or deep.
Also, most septic tanks are virtually "sealed," with a "tight-line" heading off to your drain-field. There shouldn't be much - if anything - to draw them down around it. Just remember
where the tanks lid is - they occasionally need to be pumped out!