This has def been asked before

Breath skills is when you get hit in the face. VF and BF is when you hit back.

If you run a breath skill along with VF, you get quick riposte that stops at 50% hp instead of 70%.

If you run a breath skill with BF and qr seal, you get bonus cooldown on both phases while also attacking twice on both phases.

Breath skill give cooldown reduction on all attacks on Enemy Phase. VF give's CDR on unit attacks on enemy phase.

The cooldown reduction would be handled by the Breath skill (since it procs more often) and VF would just be a 50% health threshold QR

Basically turns vengeful fighter into better QR.

Vengeful fighter normally gives extra charges when counterattacking (and only on counterattacks, not when you take hits), but if you're using a breath skill you already get extra charges, so the only reason vengeful fighter is the choice of skill is because the HP threshold is more generous.