VINCI’s balance‑sheet shows a solid but moderately leveraged position (net debt ≈ 21 bn €, net‑debt/EBITDA ≈ 2.1×, interest coverage >10×).  Senior debt can be raised at very low cost (iShares Euro Corp Bond yields ≈ 1.1 % in 2022), while hybrid (sub‑senior) bonds currently yield around 2.3 % – higher but still cheaper than equity.  Hybrid instruments receive 50 % equity credit from rating agencies, which helps keep leverage metrics within investment‑grade tolerances when the group finances large concession investments.  Because the incremental after‑tax cost of hybrids (~1.7 %) is meaningful relative to senior debt, a modest share is optimal: it captures the rating and flexibility benefits without unduly raising the overall cost of capital.  A balanced allocation would keep hybrid financing at roughly one‑quarter of total debt.

<answer>25%</answer>