How is this accomplished? The side quests don't seem to end and I've already completed the main quest line. I've done each of Vex's and Devlin's quest 3 times and and working through round 4. Is it bugged or do you really just have to go through them until they run out?

You need to complete 3 or 4 jobs in each major city to get a special job for that city. When all special jobs are done, you get to be the master

It's five jobs. And jobs are random so it can take some time before you get the last one in the city you want.

Well that explains it.

Thank you for the replies

you can also refuse jobs once they are assigned to you. they will clog up your quest log and reduce the number of quests completed in your stats (i once had -18 quests completed for TG) so if that kind of thing bothers you, it's probably a lot easier to just do the job. also, riften jobs are pointless, there is no special assignment.