At the end of the day, what reduces chances of hearing loss is a combination of moderate listening level and some awareness about exposure duration(music and any other noises throughout the day). The loudest and the longest the session, the worst it is to the ear. Everything else is so far behind in term of impact that nobody should ever neglect those 2 variables by thinking a third one protects us.
If you have a vent, there is a reduction in the so called piston effect which in turn means less force put onto the eardrum, but if we just increase the volume to feel the same listening level, I don't know what benefit is left?
In principle it's probably good to have a vent, If only for when you insert and pull out the IEM, or if you don't manage to equalize the pressure after insertion(always try to, pull on the ear lob, move your jaw, use foam, anything goes if it breaks the seal long enough to equalize pressure. Also, insert and pull out gently!!!!!).
Over time those strong pressure event are probably not helping the ear, But I don't have data on how much actual pressure is applied and if it's bad. Short loud impacts need to be really massive to be considered damaging by all the safety research guys.
The other side of the coin is that with a vent, you're likely to hear more of the outside noises which in turn usually make you want to listen to music louder to cover up that noise. So I'd say that in a noisy environment, not listening to music and wearing IEMs or earplugs is the best choice, and listening to music using sealed IEMs with strong isolation would be the second best choice.
In a quiet place, vented designs would be my choice, if only because they often are objectively superior(noise floor from outside noises excluded).
