In DList.cpp you say this:
vector<string> splitLine(char a_line[], char delimiter)
instead of this
vector<string> DList::splitLine(char a_line[], char delimiter)
It looks like a minor oversight, you are scoping the rest of the functions just fine in the .cpp file. Sometimes it takes an hour just to find the smallest typo :-)
For future reference, try to start understanding compiler errors (well, Linker in this case), because yours was actually quite descriptive:
__thiscall DList::splitLine(char * const,char)
Should tip you off that the issue was with the splitLine
function, which might have pointed you in the right direction.