In virtuemart the order number is generated prior to the payment process/ confirmation.
When a customer decides NOT to pay and cancel the order the order number is already created.

(for tax purposes the invoice number must be sequential).The order number cannot be the invoice number .
When a customer cancels the order there will be a gap in the invoice number.

customer status order# invoice#
A confirmed 1 1
B pending 2 x
C confirmed 3 2
D cancelled 4 x
B confirmed 2 3
E confirmed 5 4
etc

is this possible with VM invoice?

Hello,

yes, it is possible.
Invoices generated by VM Invoice can have own numbering sequence, independent on VirtueMart order numbering.

This may be set in VM Invoice / Configuration / Invoice Configuration.
Set Invoice Number to Own numbering.

Then you can also configure numbering prefix (Numbering prefix) and value, where numbering should start (Start Number).