i tried to generate a vanity addy and after i imported it in BLOCKCHAIN ..but the private key i imported is not really pricate as i am taking it from the vanitygen website ? am i wrong ?
There is no "vanitygen website", so you might want to clarify that.
In terms of blockchain.info and private keys, they use an encryption scheme where they shouldn't actually ever have your private keys directly. However, this does come with a level of trust - trust that their javascript hasn't been messed with, for example (they offer a browser extension as a way of keeping that separate, for the slightly more paranoid) - as it does with any wallet service (websites and executable binaries alike).
Unless you want to use the address in a transaction, you can always add it as a "watch only" address in blockchain.info .