Hi,
Please make sure you have only 1 published payment method in the "Virtuemart Configuration".
Probably you have more than one payment method published but only one of them is configured and appeared in the front-end.
go to "VM Backend >> Shop >> Payment Method" and unpublished the unnecessary payment methods and leave only one. Then that payment method will be selected automatically.
AND MAKE SURE THAT PAYMENT METHOD HAS NO RESTRICTIONS AT ALL. For example, don't select "specific country". It should be available to everybody. Otherwise, that payment method won't be selected until visitor provides his/her country (either by log gin in or registering).
Regards.