Use Style.FlowAlign or Style.FlowAlignChildren, in your sample use either of those lines:
rt1.Style.FlowAlign = FlowAlignEnum.Far
or
parentRenderContainer.Style.FlowAlignChildren = FlowAlignEnum.Far
Setting TextAlignHorz only affects text within its own space - i.e. in a situation like this:
dim rt as New RenderText("text")
rt.Style.TextAlignHorz = AlignHorzEnum.Right
rt.Width = "10cm"
the text will align to the right of the 10cm wide RenderText object, but it won't affect the alignment of one render object within another (as you tried to do in your code).
Hope this makes sense.