I live in Dongducheon and haven't had any vaxes other than tetanus since early childhood. I've never been vaxed against any of the things (Hep A/B, JE, etc.) that the military claims you should be vaxed against in order to live in S. Korea. I actually lived in a rural town near Dongducheon for four years before hubby and I met and never had any health issues aside from normal stuff (colds, bronchitis, gastroenteritis on a couple of occasions, etc.).
When hubby and I decided to move back here, we debated whether to start vaxing DD (now 11 1/2 months old) and decided that an uncompromised immune system and extended nursing would offer her more protection than any assortment of shots.
We are NOT here with the military. That said, hubby was in the Army and stationed over here when we met, and I changed to a dependent visa after we got married. The powers that were groused about my vax status a couple of times, but never forced the issue. (And FTR, I was never command-sponsored, but they still gave him permission to live off-post with me, gave him BAH for off-post housing/utilities, increased his commissary allotment to reflect household size, etc.)
ETA: Of the things the Army warns about, Hep A is by far the most "real" threat. In addition to the communal nature of dining in Korea (with shared side dishes, a common grill in the center of the table, etc.) some restaurants "salvage" leftover side dishes and re-serve them to other customers. This is illegal, but not closely monitored. So when you're eating out, there's at least a chance that you're dipping your chopsticks in the same food that Kim, Lee, Park, and Hyun did before you. Hep B is fairly prevalent as well, but is generally spread through the coffee girls, drinky girls, etc. Not a real concern for me, as my infant won't be among their patrons.