For general hypersonic vehicles flying at high altitudes and Mach numbers, the appearance of the large boundary layer displacement thickness can change the pressure distribution and aerodynamic characteristics significantly. As for the waverider, another side effect is that the shock wave position is deflected downward evidently even at the design Mach number, which is adverse for the shock wave being attached to the leading edge and may lead to more leakage of high pressure gas from the lower surface onto the upper surface. Therefore, this paper first develops a vorticity-based method to determine the boundary layer displacement thickness, in combination with the tangent wedge/cone method. Then, trying to alleviate the high pressure gas leakage near the leading edge, modification of a viscous optimized waverider is conducted under the condition of strong viscous interaction, by deducting the corresponding boundary layer displacement thickness from the original lower surface along the normal direction. Results show that the shock wave position around the lower surface of the modified waverider under the condition of strong viscous interaction is very close to that of the inviscid basic flowfield around the original waverider, which means less leakage of high pressure gas. But it's found that such change has little influence on the aerodynamic characteristics of the upper surface. However, an interesting discovery is that due to the lower pressure near the leading edge of the modified lower surface, the wave drag is lowered for the same lift, thus the lift-to-drag ratio is improved. The modified waverider also exhibits higher lift-to-drag ratio at large angles of attack when compared to waveriders with upper expansion surfaces. Overall, a vorticity-based boundary layer displacement thickness determination method is proposed in this paper, which is then used to modify waveriders to achieve higher aerodynamic efficiency.