To determine the appropriate extent to which VINCI should rely on hybrid bonds, we must analyze the company's financial health, cost of capital environment, and strategic needs based on the provided 2022 annual report data and market conditions.

**1. Financial Strength and Credit Profile:**
*   **Profitability:** VINCI demonstrates strong and growing profitability. Profit attributable to owners increased from €2,597 million in 2021 to €4,259 million in 2022. Operating profit also rose significantly.
*   **Cash Flow:** The company generates robust operating cash flows (€9,387 million in 2022), which comfortably covers interest payments and capital expenditures.
*   **Leverage:** Total Equity is €29,409 million, while Total Liabilities are approximately €82,582 million (Assets €111,991m - Equity €29,409m). The debt-to-equity ratio is moderate. The company has significant non-current borrowings (€23,630 million in bonds and other borrowings), but its strong cash generation supports this level of debt.
*   **Credit Rating Implication:** VINCI is typically rated investment grade (IG). The market data provided includes "IBOXX EUR NON-FINANCIAL IG" spreads, which is the relevant benchmark.

**2. Cost of Capital and Market Environment (2022):**
*   **Interest Rates:** 2022 saw a sharp rise in interest rates. The 10Y Swap Average rose from 0.053% in 2021 to 1.927% in 2022. This increases the cost of all debt, including hybrids.
*   **Credit Spreads:** The spread for IG non-financial corporates (sub-senior delta adjusted) averaged 2.295% in 2022, up from 1.298% in 2021.
*   **Hybrid Bond Cost:** Hybrid bonds (subordinated, perpetual or long-dated) carry a higher coupon than senior debt due to their subordination and equity-like features. In a rising rate environment, the absolute cost of issuing hybrids becomes expensive. However, they offer tax shields (interest is often tax-deductible, though regulations vary) and count towards equity for rating agencies (usually 50% equity credit).

**3. Strategic Role of Hybrid Bonds:**
*   **Equity Credit:** Rating agencies typically grant 50% equity credit to hybrid bonds. This helps optimize the capital structure by lowering reported leverage ratios without diluting existing shareholders.
*   **Financial Flexibility:** Hybrids often have deferrable coupons, providing flexibility in times of stress, although VINCI's strong cash flow makes this less critical.
*   **Optimal Mix:** Pure equity (100% equity) is the most expensive form of capital due to the equity risk premium. Pure debt (0% hybrids) maximizes tax shields but increases financial distress risk and leverage ratios. A moderate use of hybrids allows a company to lower its Weighted Average Cost of Capital (WACC) by balancing the tax benefits of debt with the lower required return of debt vs. equity, while maintaining investment-grade ratings.

**4. Evaluating the Options:**
*   **0%:** Relying solely on senior debt and equity might result in higher reported leverage, potentially pressuring credit ratings if debt levels grow. It misses the opportunity to optimize the capital structure with instruments that offer equity credit.
*   **100%:** Relying entirely on hybrids for the debt portion is inefficient. Hybrids are more expensive than senior debt. A company with VINCI's strong cash flow should utilize cheaper senior debt first.
*   **75%:** This is a very high reliance on hybrids. It would significantly increase the average cost of debt compared to a mix with senior debt. It is typically reserved for companies with weaker credit profiles or specific regulatory capital needs (like banks/insurers), which VINCI is not.
*   **25%:** This represents a conservative but strategic use of hybrids. It provides some equity credit to support ratings and leverage metrics without exposing the company to the higher costs associated with a larger hybrid book. Given VINCI's strong investment-grade status and ability to access senior debt markets easily, a smaller portion of hybrid capital is sufficient to optimize the structure.
*   **50%:** This is a more aggressive optimization. While it maximizes equity credit, the high interest rate environment of 2022 makes the high coupons on hybrids less attractive compared to senior debt.

**Conclusion:**
VINCI is a high-quality, cash-generative industrial company. In a rising rate environment (2022), the cost of hybrids increases. The company benefits from maintaining a strong investment-grade rating. A modest allocation to hybrid bonds allows for leverage optimization (equity credit) without incurring excessive interest costs. A 25% reliance on hybrids (as a portion of the total capital structure or debt mix) is a prudent, balanced approach for a company of this caliber, avoiding the high costs of 50-75% hybrid usage while still gaining the structural benefits over 0%. Most large IG industrials maintain a small to moderate portion of subordinated/hybrid debt. Given the specific options, 25% is the most reasonable "optimization" level that doesn't overpay for capital in a high-rate year.

<answer>25%</answer>