When a RadSlider is in Lightweight render mode and it is configured to be vertically oriented, its decrease and increase handles are positioned outside of the container of the control. As a result they could appear in elements that are located above and below the slider.
Currently this can be avoided if you add margin to the slider:
margin: 2em 0;
<telerik:RadSlider ID="RadSlider1" CssClass="rslVerticalSlider" runat="server"