Where in memory is vtable stored?

Depends on compiler.

In VC++, the vtable pointer stored at the beginning of the object allocation, before any member data. (Provided your class has at least one virtual member function.)

There also may be multiple vtable pointers, if your class multiply-inherits from other classes with vtables.

The vtables themselves are statically allocated somewhere in your address space.

Then the object layout looks like (for an instance of C):

A's VTable ptr
A's member variables.
B's Vtable ptr
B's member variables.
C's member variables.

for the heirarchy

class A {
  virtual Ax() {}
  int a, b;
};
class B {
  virtual Bx() {}
  int c, d;
};
class C : public A, public B {
  int foo, bar;
};