On some platforms, such as TOS, MQ VBT and MQ VSI Pro may disappear from the charts when viewing an index, like SPX. You can confirm that MQ VBT and/or MQ VSI Prois still added to the chart via the Edit Studies dialog, even though it’s not showing up.
The reason this happens is because SPX isn’t a stock like AAPL, or a Futures symbol like Crude Oil. It's a calculated value based on other symbols, and it isn't traded. Think of it more like a moving average that's based on the weighted average of the S&P 500 members' prices. Since it's not traded, it doesn’t have volume and other characteristics that MQ VBT and MQ VSI Pro require. This prevents MQ VBT and MQ VSI Pro from appearing on specific symbols like these, since TOS just hides indicators on some charts when they request non-available Volume info.
You may ask, "but aren't we trading SPX when we're trading 11-Hour Options, etc.?" When we buy or sell things relating to the SPX, we're actually buying or selling the options on SPX, not SPX itself.
To have MQ VBT or MQ VSI Pro return, change to another traded symbol, like AAPL, and you should once again see MQ VBT appear once again. You may also get them to appear on some higher-timeframe charts, like the daily chart.
Why do MQ VBT and MQ VSI Pro work with SPX on some platforms, like TradeStation?
Some platforms, like TradeStation, count the up and down price changes to come up with a pseudo-volume, even though it's not actually traded and has no trading volume. So VBT and VSI can work on TradeStation for that reason while it's not showing up on other platforms. In this case, TradeStation's pseudo-volume reflects volume in terms of price changes, but not necessarily the amount of trades being made at a price level. For "real" volume to be shown, it needs to be a symbol that's actually traded.