the purpose of PI ACE is to build modules that run on a time or event based schedule. Regarding the reuse of the PI ACE modules you might want to revisit your post here. Regarding the Matlab question you should take a look at the whitepaper Using PI Data with MATLAB that was posted recently on the Library.
For complex calculations, you should be able to do almost any calculation from VBA by including the PI SDK. What you will be missing is the ease of use of PI ACE, the scheduling and features that differentiate the VB6 alike VBA and the .NET environment. You might want to take a look at the Deeper in PI SDK (Calculations, Event Pipes and Application Identity) webinar, which explains how to access the build-in calculation functions in PI SDK.