I recently trapped a Zombie Villager, and then immediately cured him, using the old weakness-splash-potion-and-golden-apple "one-two" trick. I stayed nearby until he turned back into a regular villager, then blocked him off (for safe keeping) until I needed him for future use.
I went off, and did my thing (only 34 blocks away - yes I counted) and when I came back, he was gone! I thought he was secure, so I don't think any mobs could get to him, but could he have despawned, due to his origin as a Zombie Villager? Do I need to name my Zombie Villager? Or is it possible that a mob (perhaps a baby zombie) killed him?
As you can see, the only block that is not a solid block, is the fence gate (which I made sure was closed). This is in Vanilla Minecraft, in a single-player map.