Growing under the walls, climbing over the walls, however makes thematic sense to you. Creatures can already bypass walls by using Flying, Climbing, or Teleport so a Burrow mechanic wouldn't be much different. Mechanically though I don't see any imbalance or unfairness in the current rules, which don't care one bit about blocked passage or blocked line of sight when placing the vine markers, however you do need to have line of sight to actually cast anything from the vines or otherwise utilize them for anything other than hinderance.