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To solve this problem open the following file
components/com_virtuemart/js/themes/default/templates/product_details/includes/quantity_box_general.tpl.php
and in line 46 and 47 remove this code
<input type="button" class="quantity_box_button quantity_box_button_up" onclick="var qty_el = this.parentNode.elements[\'quantity[]\']; var qty = qty_el.value; if( !isNaN( qty )) qty_el.value++;return false;" />
<input type="button" class="quantity_box_button quantity_box_button_down" onclick="var qty_el = this.parentNode.elements[\'quantity[]\']; var qty = qty_el.value; if( !isNaN( qty ) && qty > 0 ) qty_el.value--;return false;" />and replace it with this code
<input type="button" class="quantity_box_button quantity_box_button_up" onclick="var qty_el = document.getElementById(\'quantity'.$prod_id.'\'); var qty = qty_el.value; if( !isNaN( qty )) qty_el.value++;return false;" />
<input type="button" class="quantity_box_button quantity_box_button_down" onclick="var qty_el = document.getElementById(\'quantity'.$prod_id.'\'); var qty = qty_el.value; if( !isNaN( qty ) && qty > 0 ) qty_el.value--;return false;" />