Hi,
This is a new feature comes with early Virtuemart versions. It is the "express paypal" button, so visitors can click that link and purchase the product through paypal without registering or providing any billing info on your website. But it includes lots of bugs even in original Virtuemart though.
That button shows off if you choose "Express Checkout" option for "PayPal Payment Methods" through PayPal settings. Open the VM Backend >? Payment methods, find the "Paypal" payment method and through parameters; choose "Standard" as "Paypal product" (don't choose Express)
Regards.