simple answer is: don't set the rows attribute an just use vflex="1"

<zk>
  <vlayout height="100%">
    before
    <!-- textbox takes remaining space between before an after-->
    <textbox vflex="1" multiline="true"/> 
    after
  </vlayout>
</zk>

is there a reason why you need to set both?

simple answer is: don't set the rows rows attribute an just use vflex="1"

<zk>
  <vlayout height="100%">
    before
    <!-- textbox takes remaining space between before an after-->
    <textbox vflex="1" multiline="true"/> 
    after
  </vlayout>
</zk>

is there a reason why you need to set both?