To determine the optimal extent to which VINCI should utilize hybrid bonds relative to S&P Global Ratings' cap, we must analyze the company's capital structure and cost of capital, aiming to minimize the Weighted Average Cost of Capital (WACC). 

S&P Global Ratings allows up to 15% of a company's total adjusted capital (Debt + Equity) to be comprised of hybrid bonds while still receiving equity credit (typically 50% equity / 50% debt). 

For VINCI at the end of 2022:
- **Equity** (Total Equity): €29.41 billion
- **Debt** (Short-term Borrowings + Non-current Bonds + Other Non-current Borrowings): €6.37B + €20.43B + €3.21B = ~€30.01 billion
- **Total Adjusted Capital**: ~€59.42 billion

In corporate finance, WACC is minimized by utilizing the cheapest sources of funding available. Because hybrid bonds rank senior to common equity but subordinate to senior debt, their yield inherently rests between the cost of senior debt and the cost of equity. In 2022, market data shows benchmark yields for investment-grade corporate bonds (e.g., iShares Core Euro Corp Bond yield of 1.085%) and the subordinated-senior delta (approx. 2.295%), meaning the cost of hybrid debt for a solid investment-grade issuer like VINCI is well below its cost of equity (historically demanding a significant equity risk premium).

Because the cost of hybrid capital is strictly lower than the cost of common equity, replacing expensive equity with cheaper hybrid capital—without compromising the credit rating (by keeping within S&P’s 15% equity-credit limit)—will continuously lower the firm's WACC. Therefore, to maximize shareholder value and minimize capital costs, the company should fully utilize this bucket. 

The optimal usage is to maximize the allowed capacity.

<answer>100%</answer>