It looks like you are trying to create your own vector class named Vector (keeping notice of the capitalized 'V').
These Vector method definitions need a definition for the Vector class, which is presumably in your Vector.hpp. I would like to note that the .hpp extension indicates a template header library, and all of the method definitions should be inline in the template class definition, or in an include file; not in a compiled object file.
It appears (at least from this lone file) that you are not using std::vector at all, and you may wish to remove the '#include <vector>' line to keep things clean in your code and not include not used pieces of code.
Back to your example not working, your #include "Vector.hpp" must point to the file that presumably contains the class definition. for this include to work as stated, the Vector.hpp file will have to be in the same directory as the file that you posted; otherwise you will have to give a relative (or absolute) path to this file in the include statement.
I hope this helps you some!
Michael
P.S. Why are you not using std::vector?