One Page Checkout for Virtuemart 2 loads from it's helper plugin and to disable all functionality of OPC you only need to disable the plugin or check the appropriate checkbox within the OPC backend.
One Page checkout 1 can run in component mode, where you can create a checkout link from your menu. The most common way of running this component is to install it over you basket.php file in Virtuemart installation, so you don't need to change any other links from your page to shop.cart page of Virtuemart. If your Virtuemart has any special hacks which are not inbuilt in this component and they modify ps_checkout.php we will need to merge them with our own file ps_onepage.php in order for them to work. Usually credit card processors modify ps_checkout for them to work. This modification does not require you to buy extended support.