What skills should I sharpen before my summer internship?

Honestly, you don't need to know much because they'll teach you. They'll go through derivatives and Interest Rate Swaps during the week of training so it might be redundant if you read about it now. For more technical desks, you just need to be prepared to read the primers they give you asap to be able to understand which articles might be worth including in your morning note. 

I only wish I knew VBA before starting.

You probably won’t add value and they don’t expect to you either. Obviously have the basics, know how excel works and how to be efficient in it. Know the basics that comes with the job. The main thing, in my opinion, is to build a strong EQ. If you don’t know how to handle yourself in a social setting then you can get screwed with the return offer.