We don't do verticals since those are directional trades. Bufferfly and IC are vega negative trades, calendars are vega positive. There are no firm rules, but I personally don't do IC when RVX is below 25. Just my personal rule of tumb. If you do SPY calendar, it should be a good trade with VIX around 16-17.
In general, you want to do a calendar when the IV is in the low third of the recent range.