Hi,
If you want to preserve the LightMart in Virtuemart related pages, but only want to remove its effectiveness in checkout page, then only rename this folder:
/templates/flexible_lightmart/html/com_virtuemart/cart
to
/templates/flexible_lightmart/html/com_virtuemart/cart2
Then LightMart will be inactive in Virtuemart Checkout page, and you will get back to official Virtuemart Checkout page layout. Now you can install a 3rd party one-page checkout plugin which won't generate any Conflict issue because now you are using original Virtuemart layout in shopping cart page.
Regards.