There are 2 different meanings of P2P, and you seem to be getting confused.
P2P can mean Point to Point - as is the case with a VPN client like OpenVPN or PPTP. These connections occur directly - and only between 2 devices.
It can also mean Peer to Peer, as in the case of bit torrent etc, where there is a web of connections. The rest of this post deals with these cases, as they seem to be what is of interest to you.
There is no rule for how a P2P networks can be made - it depends on the protocol. That said, there are usually a number of servers and a number of clients.
The servers role is typically to help the clients find each other.
Once the clients have found each other, they normally communicate directly - traffic between the clients DOES NOT pass through the server (and indeed, if it did it would not be a P2P network).
Depending on the type of traffic though, there does not need to be a server or list of servers - in limited circumstances its possible for all clients to advertise their presence - although this is not common across the wider Internet as it does not scale well.
An IRC server is a simple, old-style chat server - it is possible for a P2P network to use this to co-ordinate communication between clients, but this is unnecessary, and not common. (IRC servers are often used to control botnets while hiding the controllers details). A tracker is a more common mechanism for handling this function.
I'm not an expert on Bitcoin, but I understand that no tracker/central point is required - indeed one of the advantages of it is totally decentralised (ie does not require a central point or tracker) - and that each bitcoin has a chain (ie can be tracked back to its original creation).
A Bitcoin tracker is not actually required for Bitcoin, and is, AFAIK a way of finding out the value of bitcoin based on trades being made.
Best Answer
The peers don't need to know your real IP, you are giving them a way to contact you by simply contacting them yourself.
Even if the tracker shares an unreachable IP (your VPN) and other peers fail to connect, directly at least, you make yourself reachable by contacting those peers yourself.
You may be blocking inbound requests from unknown hosts, but by contacting a peer and requesting data from it yourself you are initiating a two way data connection that they can use to not only send data, but to request it as well.
The VPN is probably doing exactly what you expect, blocking unknown host connections, but once you contact someone through it you have effectively established a two way pipe between you and a peer. Whenever your software gets an updated list of peers and contacts new peers then you will get new data flowing outwards as well as inwards.
Most home router firewalls (with UPNP disabled) will automatically block incoming connections as well which creates this same problem of peers not being able to connect to you. Once you start connecting to them (per the list supplied by your tracker) then you are effectively poking very specific holes in your firewall for communication to happen to (and from) very specific places. The VPN is essentially a remote firewall from this perspective.