This one's a very situational ability, because its effectiveness depends directly on how many adjacencies there are between your vertices and enemy vertices. Usually such adjacencies are short-lived, because the faces on either side will attack each other - generally territories stabilise to solid regions of faces separated by a gap of at least one vertex, a kind of 'no-mans land' between. So often Vertex Attack will have a fairly small effect, but you can control it, try to set up situations to maximise it. It'll also depend on the level - if there are mostly open faces, there will be more adjacency because the faces aren't attacking to get rid of adjacent enemies; on some levels like the torus there is generally a lower ratio of perimeter to amount of territory, so less adjacency.
I've just run a couple of tests to measure the effectiveness on a few different levels. In a game on a sphere, all faces initially present, Vertex Attack made an average of 12 attacks each time it was used (about 24 points of damage, as each attack does 2 damage on a hit), but this ranged from 1 attack on a very poorly timed use to over 20 when large battles were going on, and over 50 when Expand Edges had recently been used. On the torus, with most faces closed, an average of 10 attacks - slightly less, because of the smaller perimeters. On a sphere with all faces initially open, an average of 20 - it's a very powerful ability in this case; one time it made 54 attacks without Expand Edges even having been used!
So it seems rather powerful for such a low level ability; situationally it's doing over 100 points of damage. But I don't believe it's overpowered; its average damage is in the mid-twenties, which is comparable to Roaming Mine or Boobytrap, and it's only doing the really high damage on specific unusual levels or in a combo with Expand Edges - and combos with white abilities can be expected to be strong, since Damage All is regularly throwing out hundreds of damage points on its own. Plus it'll sometimes do almost nothing, and you don't always know when it'll be most effective because the adjacencies might be out of sight.