That is up to the "firstbits" algo - and for the sample you gave "1btc" (firstbits are case-insensitive) is not surprisingly already taken (by 1BtCpSAJ14d64ZHTqc6uqaLMVXoovTsSXr back in 2009 by a miner and it still has 50 BTC).
To test whether a vanity address you have created is "already taken" (in regards to the "firstbits") then type in the relevant prefix (1btc in your example) into the Search in blockchain.info.
Note also that until you have sent BTC to your address (and it has been confirmed enough times) the "firstbits" are still "open" for anyone to take (first come, first served).