To reproduce:
Public Class Form1
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Me.RadVirtualGrid1.SelectionMode = Telerik.WinControls.UI.VirtualGridSelectionMode.CellSelect
        Me.RadVirtualGrid1.MultiSelect = True
 
        Me.RadVirtualGrid1.RowCount = 10
        Me.RadVirtualGrid1.ColumnCount = 5
 
        Me.RadVirtualGrid1.SelectAll()
 
        Debug.Assert(Me.RadVirtualGrid1.Selection.MaxColumnIndex = Me.RadVirtualGrid1.ColumnCount - 1)
        Debug.Assert(Me.RadVirtualGrid1.Selection.MaxRowIndex = Me.RadVirtualGrid1.RowCount - 1)
    End Sub
End Class


Workaround:
Private Sub RadVirtualGrid1_SelectionChanging(ByVal sender As Object, ByVal e As VirtualGridSelectionChangingEventArgs)
	If e.SelectionAction = VirtualGridSelectionAction.SelectAll Then
		e.Cancel = True
		radVirtualGrid1.VirtualGridElement.Selection.BeginSelection(0,0, radVirtualGrid1.MasterViewInfo, False)
		radVirtualGrid1.VirtualGridElement.Selection.ExtendCurrentRegion(radVirtualGrid1.RowCount-1,radVirtualGrid1.ColumnCount -1)
	End If
End Sub