Impedance mismatch vs Signal attenuation
The impedance mismatch doesn't cause actual loss, it just makes some of the power reflect away from the load. You are interested in a parameter called "reflection loss" that can be calculated.  \$\Gamma\$ is the reflection coefficient that tells us how much field (current or voltage) is reflected. 

$$\Gamma = (Z_l-Z_0)/(Z_l+Z_0)$$

\$|\Gamma|^2\$ is the amount of power reflected. \$1-|\Gamma|^2\$ is the rest, i.e., amount of power lost due to reflection. From that, you can calculate the amount of loss due to reflection in dB:

$$L_{refl} = -10 \times log_{10}(1-|\Gamma|^2)$$

I assumed that the \$Z_l\$ is real and plotted the reflection loss with Python for that special case. For numbers with a complex load you can use the equations above, or Smith chart as suggested by @The Photon.

If the transmission line is lossy, the high VSWR can cause extra loss, as the power loss in a transmission line is proportional to the square of the current.