fix the key of __transient_vector

the key should be per not d

flopy/export/vtk.py

@@ -1489,7 +1489,7 @@ def write(self, f: Union[str, os.PathLike], kper=None):
                                 self.add_array(array, name)
 
                             if per in self.__transient_vector:
-                                d = self.__transient_vector[d]
+                                d = self.__transient_vector[per]
                                 for name, vector in d.items():
                                     self.add_vector(vector, name)