1. 方法1
<Separator BorderBrush="Gray"><Separator.LayoutTransform><RotateTransform Angle="90" /></Separator.LayoutTransform>
</Separator>
2. 方法2
<Separator Style="{StaticResource {x:Static ToolBar.SeparatorStyleKey}}" BorderBrush="Gray" />
参考
WPF如何制作一个竖直的separator