All the efforts to reveal the damage/heal mechanics seem to confirm that weapon damage is essentially the primary stat of importance. Get a new weapon and you'll find your heals to be much more effective. For the more detailed discussion, reference this fantastic post on the topic.

That aside, as others have said, careful enmity management is integral to Stone Vigil, particularly since the Aevis mobs cannot be slept. Your DPS should be making sure they target the same mob as the tank, the tank should be maintaining aggro on all mobs, and you should avoid spamming Medica if at all possible (AoE heals pull tons of aggro).