On some platforms, such as TOS, MQ VBT may disappear from the charts when viewing an index, like SPX. You can confirm that MQ VBT is still added to the chart via the Edit Studies dialog, even though it’s not showing up.
The reason this happens is because SPX really isn’t a stock like AAPL. It's a calculated value based on other symbols, and it isn't traded. Therefore, it doesn’t have volume and other characteristics that MQ VBT requires. This prevents MQ VBT from appearing on specific symbols like these.
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 the SPX, not the SPX itself.
To have MQ VBT return, change to another traded symbol, like AAPL, and you should once again see MQ VBT appear once again.
Why does VBT 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 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.