Normally, if someone writes libelous material about somebody online, they are traced by name and/or IP address. But, let’s say there are neither. So, someone wants to accuse someone of using meth on Reddit. This user has been active for quite some time and never revealed his or her IP. Then, this user says that the actor, politician, or whoever it may be is using meth. Reddit removes the content but it keeps spreading. Could the original uploader be sued if the VPN refuses to listen?
Finding the person to prosecute is difficult but that is one of the major functions of law enforcement agencies. ISPs and VPN providers are, in some jurisdictions, required to keep metadata which is available to law enforcement either with or without a warrant depending on local law. Law enforcement agencies have varying levels of cooperation across jurisdictions - so the US FBI can request information from the UK Metropolitan Police who can request a warrant from a UK court to access a UK VPN company’s metadata and provide that to the FBI.
If they can find you, arrest you and extradite you, they can prosecute you.
In theory, private actors can find and sue people as well although the generally lack the resources and power to compel action that state actors do.
It is possible to commence a lawsuit against a known but unidentified defendant, for example, the unidentified person who posted this on Reddit. Having commenced such a case they can then seek to subpoena whatever information Reddit, say, has about the person. Failure to comply with a subpoena is contempt of court.
This whole process is harder and less certain and generally not worth it - particularly when, if you ultimately find the person they are in a country which won’t honour the judgements of your country’s courts or they have no money to pay damages.