You can set the CommandName for the button as DeleteSelected if the button is placed in CommandItemTemplate of grid. This is shown in the following demo. Command Item
Also you can try the following code to achieve the same, instead of setting CommandName.
CS:
ProtectedSub Button2_Click(ByVal sender AsObject, ByVal e As EventArgs)
Dim item As GridDataItem = DirectCast(RadGrid1.SelectedItems(0), GridDataItem)