In addition to @Joe Strazzere's excellent answer.
Should I already start looking for a new job (even if the other team is very happy with me) or should I take the risk and hope they indeed value my performance in the other team and let go of the idea I should follow up the VB.NET developer?
and
the business is disappointed in the progress I'm making in the VB.NET team and because of this they don't know if they want to extend my contract, even if I'm performing great in the other team.
If they valued your performance in the other team, they wouldn't be thinking of not-renewing your contract.
From your comments it sounds like you feel they should make things better, to make up for lying, your colleague, etc. That's not going to happen, companies don't do "should". The way to make it better is leave.