Currently, phonons can be obtained in a straightforward way only at Γ-point (there are other, less straight ways, to access phonons at different k-points)[1].
As the result of a phonon calculation (IBRION = 5/6/7/8), vasprun.xml will contain hessian matrix.
IBRIONtag in VASP Manual.- Quantum harmonic oscillator @ Wikiversity.