<asp:Panel ID="Panel2" runat="server" CssClass="Logo_Header_main_panel">Надпись
</asp:Panel>

.Logo_Header_main_panel{
    background-color:aqua;
    float:left;
     width:15%;
    height:100%;
    display:flex;
    align-content:center;
    vertical-align:central;
}

<asp:Content ID="Content5" runat="server" contentplaceholderid="cphSCCustom">
    <asp:Panel ID="panCategories" runat="server">
        <asp:Label ID="Label3" runat="server" Text="Label in panel"></asp:Label>
    </asp:Panel>
</asp:Content>

            <div id="rightcolumn">
                <asp:ContentPlaceHolder ID="cphServicecontent" runat="server">
                    <div id="servicetags" style="text-align:center;">
                        <div class="spacer">&nbsp;</div>
                        <asp:Label ID="lblServiceTags" runat="server" Text="Service Tags"></asp:Label><br />
                        <hr />
                        <asp:Button ID="cmdEnglish" runat="server" Text="Englsih" CssClass="languageButtons" UseSubmitBehavior="False" />
                        <div class="spacer">&nbsp;</div>
                        <asp:Button ID="cmdRussian" runat="server" Text="Русский" CssClass="languageButtons" UseSubmitBehavior="False" />
                        <hr />
                    </div>
                </asp:ContentPlaceHolder>
                <asp:ContentPlaceHolder ID="cphSCCustom" runat="server">

                </asp:ContentPlaceHolder>
            </div>

<div id="rightcolumn">

#rightcolumn {
    background: #99CCFF;
    width: 150px;
    margin-left: -150px;
    float: left;
    font-size: 1em;
    text-align: center;
    line-height: 1em;
}