The problem is not that someone got access to my wallet and made the transaction, but services blockchain.info as not helping to understand how it happened, in what way.
1. If your funds were accessed from their interface, the could indeed give you info like the IP where this was done.
But for that to happen, the "attacker" should have been access to your e-mail and 2FA, which doesn't have too high probability.
2. The other option is that the attacker got access to your wallet's private key or seed. In this case he rebuilt your wallet anywhere in the world without using blockchain.info service and moved away the funds. And then all blockchain.info could do is to acknowledge that this has happened, since whoever has the private key is the owner of the funds, no matter where it does the transaction from.
3. They may have a vulnerability they don't know about. But since they cannot be held responsible for anything... not much to do.
Just imagine an attacker has full access to your computer and/or smartphone.
You have to think what info of yours could have leaked:
1. Private key/seed?
2. E-mail and password?
3. 2FA seed?
Also booting from a "Recovery CD" made by one of the antivirus companies and full scan of your computer cold help fine tuning the possible directions where the "leak" comes from.
I hope that this helps you at all. Also, people always advise to use local wallet for good reason.