I think I read in the recently released Professional Breeders Series that male vents often thrash their rear legs when tending eggs and this may aid in liberating tadpoles from the egg sac.
With my vent eggs/tadpoles, if I see that the fine veins near the gill slits are gone, then I will help them break out of the egg sacs. I have my eggs fully submerged and often there is algae growing over the exterior of egg masses, so I wonder if this further complicates their ability to break out on their own. I do on occassion (a combination of work schedule and procrastination) allow the tadpoles to break out on their own - not all make it out.
Why not try a little experiment and split the clutch and assist one half and let the other half fend for itself.