An emergency fund should be sitting in a low-earning account where you can get at it quickly, like earning 1% in an online savings account.
If your Roth account has been around 5+ years, you can withdraw the contributions but not growth above your original contributions. You can use a Roth account as an emergency fund, but if you buy stock funds you put money at risk that you might need quickly. And money market funds earn a terrible interest rate (well under 1%).
If you park the money in S&P 500 fund at Vanguard, it earns 2% dividends right now. Keep it there 60+ days and the dividends are qualified, and the tax rate improves (10-15% IRS tax bracket becomes 0%, 25-35% becomes 15%). Even a 15% tax on a 2% dividend works out to 0.3% per year.
But again, for an emergency fund you aren't aiming for highest returns - you want safe money you can get at quickly. Online savings is fine for that.