I know that even when connected to a VPN, the ISP (or anyone who controls my WIFI network) could see that I am connected to (A VPN brand), what I would like to know is would they obtain the exact IP that I'm connected to.
As a side note, what could the ISP really see when I use VPN? I imagine packet size and frequency should be compromised, but what else? (MAC? hostname? DNS info?)
Yes, your ISP would be able to know the entry IP of the VPN you're connected to. They are the ones connecting you to that specific IP, after all. They should not be able to know the exit IP, though, and therefore should not be able to see what ultimate site you're connected to or what you're doing there (this assumes the VPN is encrypting your packets properly, of course).
In short, the ISP can see everything outside the tunnel and nothing inside. The IP headers tells the destination i.e. the other endpoint of the tunnel, and TCP or UDP headers may reveal the protocol with a standard port.
On a single VPN server the IP address on the other end of the tunnel is typically the same that is used for the connections leaving the server. With commercial VPN providers you might even be able to "switch locations", which technically means your traffic comes out on a different IP addresses i.e. gets internally routed in their systems. More importantly, it gets mixed with the traffic from other customers, increasing anonymity.
Everything from network layer L3 and up goes through all the routers between the source and the destination. That means the IP addresses, port numbers and unecrypted application layer protocols etc. are seen by every router in between. The MAC address is part of a data link layer L2 protocol and is only seen by the next node. With VPN, there's a new set of L3+ stack that goes encrypted inside the tunnel.
The ISP will have to deliver your encrypted data to some IP address, so obviously will know that IP address. That IP address may be a proxy that forwards your data to the VPN server at another IP address; if the proxy serves multiple VPN servers then it would be unknown which of a small number of VPNs you are connecting to.
Your ISP may also look at some characteristics of your encrypted traffic and find that you are connecting to a VPN and possibly which VPN.
