When you're connected to a VPN, your ISP can only tell that you're connected to a VPN server, nothing more. If you're using a public VPN service, they can probably figure out which one you're using.
If you are using a private VPN server, they can still tell you're connected to a VPN by gauging the amount of encrypted packets being transferred from your source IP to a single destination IP.
If you've configured the VPN correctly, they can't tell which websites you visit while connected to the VPN. However, if the DNS server for your VPN connection is set to your ISP's servers, they can tell which websites you visit. They still can't see what you download or upload though because that is encrypted.
You can test if your DNS servers are leaking information about you by visiting https://dnsleaktest.com after you're connected to a VPN.