Heyo.
The other week I made a thread in 'bug forum' about a vanishing villagers. Turns out that all villagers are probably slowly dying out in the server.
It's because we use mob grief off, that means that villagers can't farm or pick up food they're given. This means that if the village is loaded, they will be getting munched by zombies, yet can't breed.
I found a dude who has fixed this problem on his server, which also uses mob grief off. He is using a clock command to set all villagers 'willingness to breed' to maximum every 5 to 10 mins.
http://www.minecraftforum.net/forums/minecraft-discussion/redstone-discussion-and/commands-command-blocks-and/2785852-automatically-making-all-villagers-willing-to
I'm about to log on and try to get them going with trading. Let me know what you all think of this solution though.