[sqlite] HELP WITH SQLITE INTERNALS - VDBE and Virtual tables

[Uma Krishnan]

Hello,

I'm a student trying to understand SQLite for my DB project. There are a couple 
of aspects that I don't quite understand:
1) VDBE. I see how the vdbe stack is created using VDBEAddOp etc. But once the 
code is generated, I don't see when it's executed. Moreover, is it typical to 
have an implementation like VDBE in other databases as well?
2) VIRTUAL TABLES.  Why should the shared_cache be disabled when using VIRTUAL 
TABLES?

Thanks

Uma