You almost found the solution by yourself.
The language file is this:
ROOT/languages/en-GB/en-GB.com_virtuemart.ini
and look for this Line:
COM_VIRTUEMART_ONCHECKOUT_DEFAULT_TEXT_REGISTER="Please use <strong>%1$s</strong> to easily get access to your order history, or use <strong>%2$s</strong>"
Remove variables in it and change it to a constant text something like:
COM_VIRTUEMART_ONCHECKOUT_DEFAULT_TEXT_REGISTER="Please use <strong>Register and Checkout</strong> button below to proceed."
Or to whatever you want.
Regards.
Forum Support.