Tire settings
The Tires inspector defines the tire friction curve for the wheels in the vehicle.
The horizontal axis is slip in m/s (each vertical green line is 1 m/s). The slip velocity is the velocity the surface of the tire slides with respect to the ground. The vertical axis is the coefficient of friction for a given slip velocity.
The coefficient of friction is a function of the actual slip velocity. A small slip velocity increases the coefficient of friction up to the tire's peak friction. But if the slip gets increased beyond that peak point then the friction rapidly decreases to lower values.
This is the default tire friction curve in VPP:
 For slips < 0.5 m/s the coefficient of friction is 0.95
 For slips > 0.5 m/s the coefficient of friction increases progressively up to 1.1 at 1.5 m/s.
 For slips > 1.5 m/s the coefficient of friction decreases progressively down to 0.8 at 4 m/s
 For slips > 4 m/s the coefficient remains constant at 0.8
That tire needs quite a lot of slip for the coefficient of friction to decrease with respect to the adherent state, and the different is small. This default tire setup is very forgiving and allows a good control of the vehicle in most situations.
A competition tire looks more like this:
In this tire the peak friction (1.1) is reached at just 0.6 m/s. Any slip beyond that, and the friction rapidly drops 0.6. Real racing drivers have a great ability to apply the correct throttle to keep the tire at its maximum friction.
A more realistic friction shape can be achieved using the Pacejka friction model:
In this case Stiffness value to configures the slip where the curve reaches the peak friction at.
Converting an existing Pacejka set to VPP
Standard Pacejka sets are based on slip ratio and slip angle while friction curves in VPP are based in slip velocity. Thus, standard Pacejka coefficients are not directly compatible with VPP. Still, existing Pacejka sets may be adapted to VPP following this procedure:

Define a velocity
$V$ for the vehicle. Ideally, it should be the same velocity that was used for extracting the original Pacejka coefficients out of the real tire. If this is not available, I'd use some representative velocity from the speed range the tire is designed to. 
Use the Pacejka coefficients to draw the normalized tire friction curve, but based on
$\omega R_e  V$ (longitudinal slip velocity) or$V_x$ (lateral slip velocity) as horizontal axis, instead of slip ratio ($\sigma$ ) and slip angle ($\alpha$ ) respectively. The equivalences are:$$\begin{align} \sigma &= \frac{\omega R_e  V}{\vert{V}\vert} \\ \alpha &= \tan^{1} (\frac{V_x}{V_y}) \end{align} $$ 
Configure the tire friction in VPP to match the resulting curve as closely as possible. You may use either a Pacejka model or any of the other modes (i.e. Parametric), which are typically easier to fit to an existing curve.