I know you're not my developer. I don't have one, that's why I'm trying to learn and figure stuff out on my own, with a bit of help if possible

To answer your questions:
Before your suggestion of creating a new payment method, the customer already had some pending orders, waiting for payment. So the way it worked in the past was that they would follow all checkout steps and confirm the order, but then when PayPal window pops up, they close it. This way, the order comes into VM backend and remains on pending. This used to work just fine, until suddenly a few weeks ago, it didn't and all the orders started getting overwritten and order numbers missing. So that's why I am here and I now created the new payment method (reservation) as per your suggestion and I am very grateful for the tip.
The reason I am mentioning older orders is because the older pending order from 3 march has now been overwritten bij the new reservation order. The number of the older order is gone and replaced with new number from today.
Perhaps I need to cancel all pending orders of customer first. I will try this.
I did notice though that when the new reservation emails come in (order confirmation email) there is strange information in subject line, not like it normally is.
COM_VIRTUEMART_MAIL_SUBJ_VENDOR_L Is this something that can be fixed to show the correct subject like with regular orders which are not reservations?