You were not far from the solution. With your getArrayData definition, you can simply call it like this from VB:

Dim b As Variant
Dim result As Integer
result = mInterface.getArrayData(b)

However, you can also define it as a property in .idl, like this:

[id(2), propget] // note propget
HRESULT ArrayData([out, retval] VARIANT* pVal);

like this in C/C++ (you don't need result since the array is self descriptive):

HRESULT STDMETHODCALLTYPE get_ArrayData(VARIANT* pVal) ...

and like this in VB:

Dim a As Variant
a = mInterface.ArrayData

Here are the VB watches for a & b variants:

PS: VB(6) is old, but COM is not, it's still used everywhere in Windows :-)