How can i remove cell vertical separators only in selected row?
Have you tried reaching it programmatically? You have to clear the BorderBrush and the Thickness properties of each cell. There is a rectangle called "RightBorderLine".
e.Cell.Presenter.ClearValue(DataGridCellPresenter.BorderBrushProperty);
e.Cell.Presenter.ClearValue(DataGridCellPresenter.BorderThicknessProperty);
If you try to override the Datagrid cell style it will affect only the selected datagrid cell (the one you clicked in when selecting the row).