Disclaimer, I'm not a computer nerd, so I'm sure many people here will be able to amplify/correct this:
Yes, https encrypts your data between your browser and the server. Which is why when I'm at home, I don't use the VPN.
However, when you're on a network you don't trust, which should basically be any that isn't yours, there is a pretty high amount of traffic and peripheral information that is visible to anyone that knows how to look for it. Think of the https as you sending a message in code over the radio with both the sender and the recipient named in the clear. Anyone with a radio can hear that you are talking to site X, W, and Z. The VPN takes that same coded message, but now encrypts it a second time and all anyone can see is that you're sending data to site Y. The VPN (site Y) in turn talks to X,W, and Z, but the person monitoring the network can't see that. Its just another layer of protection and protecting information that isn't protected with https.