IF you want to use a 3rd party OnePage Checkout plugin. then you need to disable the DarkMart's effect on checkout page.
For this, find these folders:
/templates/flexible_darkmart/html/com_virtuemart/cart/
/templates/flexible_darkmart/html/com_virtuemart/user/
and rename them to:
/templates/flexible_darkmart/html/com_virtuemart/cart2/
/templates/flexible_darkmart/html/com_virtuemart/user2/
After you change those folder name; you will have the default layout of Virtuemart in the shopping cart. Then you can install any 3rd party OnePageCheckout plugin you want.
Regards.