The rivalry between these two communities has waned incredibly, but even the suggestion of a merger 30-40 years ago would have been met with derision from both sides. Growing up in Vineland, we were told from a very young age not to socialize with "Millbillies" and it often led to physical altercations with our neighbors to the south. We used to sing songs at the high school about not wanting to be like the "hicks from Millville high." It was a quaint demonstration of community pride that no longer exists with the younger generations in either town, for better or worse.
The original city of Vineland was a rather densely packed area in the center of town until it merged with Landis Township in the 1950s, making it the largest city by area in NJ. This created a unique situation as far as cities go, with an industrial, commercial, and residential landscape towards the center of town gradually giving way to woods and farms as you move outwards. I've always been fascinated by the hamlets that were gobbled up by the city merger such as South Vineland and Clayville. Millville too has its interesting historic sections such as Newcombtown (home of "Lamb Legs") and Leamings Mill. The population has steadily increased in Vineland allowing for traffic to become almost unbearable during "rush hours" on roads that were not built for the number of people in the city. That being said, Charles K. Landis was laughed at by his surveyor when showed the plans of how wide Landis Avenue was to be built. Conversely, it was just enough room for customers of the shops that lined Landis Avenue during it's economic heyday from the 1920s-50s.
Changes in production forced many of the earlier textile and glass industries to move elsewhere, leaving behind those vacant brick factories in the center of town. Another reason there are so many empty stores and buildings comes from the development of larger "box chains" in that Vineland / Millville corridor you mentioned as well as the section west of Delsea Dr. on Landis heading towards Rt. 55. The city implemented a program in the early 2000s to beautify Landis Avenue storefronts and encourage shopping at local businesses but at the same time they allowed for a huge Walmart to be built at the end of the street, siphoning off foot traffic and interest in Landis Avenue. It did not help that walking down the Avenue was not the safest endeavor either. In addition, many of the families who owned those mom and pop stores retired and / or moved out of town. To be fair, there are businesses and restaurants opening up in the city all the time that cater to the shifting demographic change in the city as well.
The Vineland area has an incredibly rich history going back before Landis arrived with his dream of building a utopia in the land of "pines and vines." Colliers, small farmers, and tar kilns dotted the area in those days (there is a Tar Kiln stream and road that cuts through the center-west portion of town). I try my best as an History teacher at the High School to impart the past to my students about the city (how progressive it was early on and the figures and characters that called this place home) and the area in general, letting them know there are woods and rivers and lakes not far away, as most of them hardly venture out of their neighborhood.
The loss of the seemingly inconsequential rivalry between these two neighbors speaks to the lessening of city pride among the denizens of this corner of Cumberland County and leaves both with a rudderless ennui, which manifests in the aforementioned downtown blight.