We have tried to dig it out, used Roundup, but to no avail. A friend recommended a weed torch, i.e. "green dragon". Might this work on periwinkle in a forested area?
Certified GKH Gardening Expert
Weed torches are great for killing off the aboveground foliage of plants, but the heat doesn't penetrate deeper than a few inches into the soil. This is good because you won't damage tree roots and other soil life, but you also won't reach the tough root system of the periwinkle.
If you come back repeatedly to flame off the foliage every time it grows back, your plan might work. You will probably have to do some manual removal of the roots as well, though.