Hi Dharap
It looks like, if you put 2 vbox side to side, each one is automatically 50% of the parent occupying and the width% is indicating how much of this 50% you want to use.
<layout>
<hbox width="1500" css="background-color:blue">
<vbox width="100%" css="background-color:black">
<textField width="100%" id="field1Field" property="field1"/>
<textField caption="test1"/>
</vbox>
<vbox width="100%" css="background-color:red">
<textField width="100%" id="field2" property="field2"/>
<textField width="100%" caption="test1"/>
<textField caption="test2"/>
<textField caption="test3"/>
<textField caption="test4"/>
</vbox>
</hbox>
</layout>
As you see, if you work with 100% on the vbox all is fine.