Hi!
I have previous experience integrating software written in different programming languages (C++ with C# and VB) using the SWIG toolkit, and also experience with VB6 COM class libraries.
I see two choices to do the required job:
1.- Write a pure VB6 DLL to invoke the APIs you require (or some more) from Spotify (faster development).
2.- Write a COM dynamic link library in Visual C++, wich exposes the functionality above (faster runtime).
For any of the two options, the DLL I will write component will have two layers. The first layer will implement raw JSON API communication, and the second one, will have the specific Spotify API wrapped from the first layer.
PS: I see you are from Spain. I'm native Spanish speaker (from Chile).
Warm Regards.