So i checked the workers and found one with a new name.
Only for 30 minutes,then the thief realized what he was doing and was off... but is there a way somehow to get this IP?
As others have said, you cannot directly get this information, but if you document it all, get screenshots, etc., and file a report with your local police, law enforcement can then contact the pool and get the IP used to connect to the pool at the time. Then they can also track down the ISP and who was using that IP at the time of connection.
I assume you reported the theft of the miners when it happened, so the police would welcome this additional information to help them track down the culprit.
You are forgetting that ViaBTC is a Chinese pool and most likely would ignore any court orders unless it was from the Chinese government. So there is a very slim chance that this will ever end up working.
If you do get the IP address, and if its some ISP from North America then you have more luck since the ISP would need to comply with local law enforcement.
However the biggest hurdle now is getting the logs from ViaBTC, which I don't see them happening.