To determine the appropriate level of hybrid bond issuance for VINCI for the next 18 months, we evaluate the entity's financial position based on the provided 2022 data.

**Financial Analysis:**
1.  **Capital Structure:** VINCI shows strong equity growth. Equity increased from €24.77B in 2022 to €29.41B in 2023. Total liabilities (excluding equity) are substantial, with significant noncurrent bonds issued (€20.43B) and short-term borrowings (€6.37B). 
2.  **Performance:** VINCI generated strong cash flows from operating activities (€9.39B in 2022) and maintains a robust cash position (€12.58B). Its Profit/Loss from ordinary operating activities increased significantly from €4.72B to €6.82B.
3.  **Investment Needs:** The company has active concession and infrastructure projects (Service Concession Rights rose to €28.22B), and it is pursuing M&A and investment strategies (purchases of property, plant, and equipment/intangibles of €2.62B, plus other investments).
4.  **Rationale for Hybrid Bonds:** While VINCI is a strong, profitable entity with a solid investment-grade profile, the capital-intensive nature of its concession business and the necessity for ongoing M&A or expansion capital suggest that maintaining balance sheet flexibility is prudent. However, given its strong cash position, low net finance costs, and high profitability, it does not face immediate "downgrade risk" or "significant leverage pressure."

**Assessment against Guidelines:**
- The company is not in a position requiring stabilization (0% or 3.75%).
- It has moderate, ongoing acquisition and infrastructure development needs, but is not undergoing a "transformational" event requiring maximum hybrid support.
- A 7.5% level aligns with "moderate refinancing or acquisition needs" where hybrid issuance provides meaningful improvement to leverage metrics and rating headroom, while the cost impact remains manageable given the current interest rate environment (rising swap rates in 2022).

Given the strong performance but the inherent capital intensity of the transportation infrastructure and concession sector, a moderate use of hybrid capital (7.5%) provides optimal flexibility for future growth without over-leveraging or incurring excessive interest costs.

<answer>7.5%</answer>