Some suggestions for improvement

[man.of.earth]

Hello,

Since I made some customized modifications to the core and I have to apply them every time VM is updated, I was wondering if you can add some features like customize order and invoice numbers. I saw that the "Advanced Order Numbers" extension is free for some time, and it's under GPL license - https://open-tools.net/virtuemart/advanced-ordernumbers.html (it seems it needs some adaptation to the latest VM version).

[StefanSTS]

The VirtueMart company iStraxx is selling automated numbers as a plugin. They need some income too, so I guess they won't fix another developers free extension.
Automated numbers (has a horrible documentation, but works reliable)
https://extensions.virtuemart.net/automated-numbers-detail

Open Tools stopped developement, so if you want a free extension, you have to either fix the extension yourself or ask someone to do it.
The above tool might be a quicker and not too expensive solution to your problem.

Regards
Stefan