Here are some pictures of prolapsed vents below. Healthy tissue is bright pink. If there is dark tissue, it may be necrotic (dead.) Sometimes there may be crust on it, and that needs to be scrubbed off gently in a bath to debride it.
a fairly normal prolapsed vent (photo by armorfirelady)
Prolapse with egg binding, egg blocking the vent
Prolapse with some dark, possibly necrotic (dead) tissue
Prolapse with necrotic tissue--needs to be debrided down to healthy tissue

a fairly normal prolapsed vent (photo by armorfirelady)

Prolapse with egg binding, egg blocking the vent

Prolapse with some dark, possibly necrotic (dead) tissue

Prolapse with necrotic tissue--needs to be debrided down to healthy tissue