@DrDocs - I’m not too technical with VBA. But I can surely help you get started with assistants. If your goal is to have a single completion, you might not require Assistant ID. But if your goal is to create a chat session with the Assistant. You need to create assistant → create thread → add messages to the thread → create run → submit outputs for completion. I’ve written a short how to guide using the Python SDK check it out, might help you get started. Hope this helps 
PS: Make sure you create Assistant once. Use that ID in your flow.