The engine library is compiled for Any CPU, so if your app is 64-bit (or Any CPU) and on a 64-bit machine running a 64-bit OS then it should be loaded as a 64-bit library.
If you really want to double-check the 64-bit status, one way would be to include the Loupe Agent in your app (you can even connect it up with the Loupe Agent for VistaDB and get logging from the engine) and use Loupe Desktop to view one of your app's sessions. Under the Session Details tab in the session viewer (after opening a specific session) you can see what the Agent determined as far as the OS Architecture and App Architecture and see the list of loaded Assemblies and what architecture they were compiled as (it should say MSIL for VistaDB, which will run in the same mode as the App Architecture). I'm not sure if the architecture can be found in the Visual Studio debugger (I'm not seeing it in the Modules list, for example).
However, a million+ rows is quite a large scale and can certainly expose limitations in scalability either in the way your queries are structured--because VistaDB cannot perform query-rewriting the way SQL Server can--or an issue in the engine affecting performance at such large scale which hadn't previously been noticed or hadn't needed addressing before.
But, to help you with the performance issue we'll need specific example queries and an example database to run them against to demonstrate the performance, so it would be best to open a Ticket by sending email to email@example.com or via the portal here (see the Tickets section next to the Forums).
Is there a way to ensure that VistaDb is functioning in 64bit?
My application is running in 64bit (environment.is64BtiProcess is True) but I want to ensure that VistaDb is having access to the available Ram as I've notices some slowdowns when we have 1,000,000+ row db's