To show Simple Attributes on the browse page, apply the following changes to Virtuemart:

After line 16 in addtocart_form.tpl.php (browse template, includes folder) add:

<?php echo $ps_product_attribute->list_advanced_attribute($product_id);?><br />

This shows the combobox for your simple attributes (for example: a color picker)

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Around line 410 in shop.browse.php (administrator component, html folder) comment/remove the following bit:

&& !ps_product::product_has_attributes( $db_browse->f('product_id'), true )

leave the rest of the if statement alive
this shows the add to cart button, that is normally hidden when you have simple attributes set up.