what are the chances of vanitygen address - private key pair collision?
Samr's vanitygen gets its initial private key from OpenSSL's random functions, which have been well vetted. Even if you specify a random seed (which is a good idea to add even more randomness) the entropy generation random libraries from the crypto is still used. It has equal randomness and strength as Bitcoin's address/key generator, with the exception that you are discarding millions of addresses that don't start with your vanity phrase.