Hey, this is what I have done. This is a required field validation for my order timing validation (calendar).
You may use this method for your needs.

First file:\components\com_virtuemart\views\cart\view.html.php
This modification is to insert the Java form validation on the cart page instead of submitting the form straight away.
if (!VmConfig::get('use_as_catalog')) {
//$checkout_link_html = '<a class="vm-button-correct" href="javascript:document.checkoutForm.submit();" ><span>' . $text . '</span></a>'; -- original code
$checkout_link_html = '<a class="vm-button-correct" href="javascript:myValidate(this);" ><span>' . $text . '</span></a>';//replaced code
} else {
$checkout_link_html = '';
}
$this->assignRef('checkout_link_html', $checkout_link_html);
}
2nd file:\html\com_virtuemart\cart\default.php - this is a template override file
Insert this java script and replace the 'ordertiming' as you want. It would be 'comment' for you.
<script language="javascript">
function myValidate(f) {
var ot = document.getElementById('ordertiming');
//document.checkoutForm.submit();
if (ot.value=="") {
alert('Please Select the Order Timing');
}
else {
document.checkoutForm.submit();
}
}
Hope this helps you guys. cheers