Simple average...Take the widely circulated regular trading session closing VIX price (to two decimals) of each CBOE trading day, M-F, excluding CBOE holidays, add them up, and divide by that number of trading days, using data from CBOE: https://www.cboe.com/tradable_products/vix/vix_historical_data/

Specifically, their data dump, if available: https://cdn.cboe.com/api/global/us_indices/daily_prices/VIX_History.csv

I will not trade.