IsolatePlus is an option for Isolate and other functions using Isolate. If it is set to True, Isolate will split sums that contain elements from vlist, to be able to abbreviate the vlist-free part.
Isolate[a + b + c + d, a] a+b+c+d
Isolate[a + b + c + d, a, IsolatePlus -> True]a+\text{KK}(19)