You're correct - applying corner options to a text frame causes the vertical justification options to gray out as unavailable. This is covered in the online help:
"Vertical justification isnt applied to text that takes on a non-rectangular shape due to influences such as text frame shape, text wrap, or corner effects. In these cases, top alignment is applied. When a corner effect is applied, vertical justification is possible if you make the text area rectangular by increasing the Inset value in the Text Frame Options dialog box, relative to the Size value in the Corner Options dialog box."
Hope that helps. For what it's worth, my experience has taught me never to muck with text frames and keep them separate from graphic elements. I would use a rounded-corner graphic frame combined with the unmodified text frame - but you may have reasons to do it your way.