Hello,
pleases change class="button" to anything else (for example class="button2" ) in your selected opc template file onepage.logged (and unlogged).tpl.php (the file is located in /components/com_virtuemart/themes/default/templates/onepage/YOUR_OPC_THEME_OR_NONE_FOR_DEFAULT/onepage.unlogged (and logged).tpl.php, or change opc templete at the display tab.
If you wanted us to to fix your CSS issues and you did not purchase the full 99 euro version of our component, we normally charge from 50 to 500 eur per custom template work. Usually 100eur per day (at least 4 hours).
Best regards,
Stan Scholtz