Correct use of Hertz Contact model

Hello every one...
i used Hertz contact model in my simulations in following form:
F=(Kn δn- γn Vn ) + (Kt δt- γt Vt )
The first term represents the normal force between two particles while the second term represents the tangential force. Here Vn and Vt are normal and tangential components of the relative velocity. The coefficients K_n, K_t, γ_n, and γ_t are calculated from the material properties.
As per my supervisor, the terms for normal and tangential forces are added as scalars. The first two terms contribute to the normal force Fn and the other two terms contribute to the tangential force. These two point in two mutually perpendicular directions and hence can not be added as scalars.
He asked me to write this equation in vector form, by using arrow as over bar.
Now i am confused where to put arrows ??? i mean which term need arrows as over bar.??
Kindly if someone can guide me regrading the same...
Thanks everyone