One of the very useful things about the Open AI API is that the response includes the amount of input and output tokens used , as well as the model that was hit. I am recording this data per call in my firebase instance for my app along with time of processing to monitor performance. I also add the appropriate cost per model in there and so I dynamically calculate the costs per model call as well. This helps me make business case decisions on how I use the AI. 
It would be great to have that  available in  the Vertex AI API calls as well. 

Is the way to get that now to make a separate call?

const { totalTokens, totalBillableCharacters } = await model.countTokens([prompt, imagePart]);