Interest Rates: Changes in prevailing interest rates, especially short-term rates, directly impact the yield and therefore the attractiveness of money market funds like VMMSX.

Credit Risk: The creditworthiness of the issuers of the securities held in the fund's portfolio (e.g., banks, corporations) affects the fund's perceived risk and thus its yield and share price.

Fund Expenses: The expense ratio (fees charged to manage the fund) reduces the net yield paid to investors, influencing its relative competitiveness and investor demand.

Supply and Demand: General market conditions and investor sentiment impacting demand for money market funds can influence the fund's share price.

Economic Conditions: Broader economic factors such as inflation and economic growth can indirectly affect interest rates and credit markets, impacting the fund.

Regulatory Changes: Changes in regulations governing money market funds can alter their risk profile and investment strategies, affecting their attractiveness to investors.