The august Big Ten, with its emphasis on academics and tradition, would never ditch Northwestern. Moreover, NU is fully competitive in athletics, including football and hoops (compare its record with Minn., Iowa, Purdue, Indiana, Rutgers, Terpville); it is the only Chicago-area school in the Big Ten:; it is the highest-ranked university in the Big Ten.
Vandy? I think it's safe, in that I don't see the SEC merging with anyone else. Being in Nashville also helps. Plus academics.
The best example of what you describe is the departure of Texas and A&M to the then Big 8, bringing Baylor and TTech along, and consigning TCU, SMU, UH, and Rice to the nether regions of college athletics. Truly, Texas had been complaining for at least a decade about the poor economics of the SWC. I personally didn't think the Big 8 was much better, except for the presence of OU.