The vector acoustic modes in both well-established and emerging designs of optical waveguides have been studied through use of a computer code which has been developed based on the finite element method (FEM). Dispersion curves and the displacement vectors for the transverse and longitudinal acoustic modes and the modal hybridness have been determined and these are shown for both low and high index contrast silica (SiO2) acoustic waveguides. Stimulated Brillouin scattering (SBS) frequencies are also reported for the subwavelength size SiO2 optical waveguides.