Compiling:

struct _Impl_base
{
  _Impl_base() = default;
  virtual ~_Impl_base() = default;
};

template<typename _Tp>
class _Impl : public _Impl_base
{ };

int main()
{
  _Impl<int> i;
  return 0;
}


Gives:
gcc version 4.4.0 20090208 (experimental) (GCC) 

%COMP.sh "-g -O0 -std=c++0x -fno-inline" vague_vs_default.cc
vague_vs_default.cc:4: warning: inline function ‘virtual _Impl_base::~_Impl_base()’ used but never defined
vague_vs_default.cc:4: warning: inline function ‘virtual _Impl_base::~_Impl_base()’ used but never defined

Which seems strange, as the base destructor is defined as default.

Also, the vauge linkage/vtable documentation bits here
http://gcc.gnu.org/onlinedocs/gcc-3.0.2/gcc_6.html#SEC118

may need to be updated or expanded to take into account defaulted destructors vs. key method vtable emission.