Consider switching to https://github.com/nuclearcat/ipt-netflow fork instead of https://github.com/aabc/ipt-netflow.

https://github.com/aabc/ipt-netflow is abandoned: last commit 9 months ago, it doesn't build with latest kernel.

https://github.com/nuclearcat/ipt-netflow among all forks has:

• Latest update (3 weeks ago, next one - 2 moths ago. And it was created 5 years ago)
• Most stars (12, next one 3)
• Has fixes for latest kernel.

So try:

1. Rebuild with this fork, give it to testing.
2. If everything works fine, submit VyOS PRs to this fork and see if maintainers are open to merging PRs.