VIX index (spot) values are the intraday values of the index only. This data is available starting from January, 1992.
The frequency of the VIX index values is every minute for data prior to 2004 and every 15 seconds, including milliseconds, from 2004 and after.
** US Central time zone until September 2019, files from October 2019 will use the US Eastern time zone and include millisecond precision in the timestamp