VINCI in 2022 shows solid profitability and sizable scale with high equity and large concession/infra footprint. Net finance costs around 614m, gross finance costs 750m, EBITDA supporting robust cash flows. However, large capex in concessions and PPPs and growing intangible/assets (Goodwill, service concession rights) imply substantial future investment needs. Hybrid bonds would help optimize capital structure by lowering cost of debt and improving adjusted leverage, but S&P cap restricts equity credit up to 15% of total adjusted capital. Starting point of hybrids is not provided; assume some hybrids exist given large European infra groups often issue hybrids. Given 2022 high capex and potential refinancing needs, and the desire to preserve investment-grade rating while not overusing hybrid issuance, a moderate approach seems prudent: aim for around 7.5% of total adjusted capital as hybrid issuance in next 18 months, which per guidelines corresponds to meaningful leverage improvement with manageable cost of debt impact and aligns with substantial capex/acquisition plans without excessive dilution/rating risk.

Final answer: <answer>7.5%</answer>