After the upgrade to FRR 10.2, a configuration option for setting the source-address for SRv6 encapsulated packets is now available:

segment-routing
 srv6
  encapsulation
    source-address 2001:db8::1
  exit
 exit
exit

This task adds this configuration options to VyOS as:

protocols 
 segment-routing {
  srv6 {
    encapsulation {
      source-address "2001:db8::1"
    }
  }
 }
}

An additional show command show segment-routing srv6 manager is added to verify:

vyos@vyos:~$ show segment-routing srv6 manager
Parameters:
  Encapsulation:
    Source Address:
      Configured: 2001:db8:1::1

Note: this depends on FRR commit #c8e83c3 (merged to master in Nov 2024). It does not yet appear to be part of any release.