What's the probability of vanity pools causing RIPEMD-160 (public key hash) collisions?
Bitcoin addresses are 160-bit numbers. Hence, randomly finding a private key corresponding to an address takes on average 2^160 attempts.

If 4 billion computers work on cracking a key doing 1 billion calculations per second for 15 billion years, they will find a total of 1.8*10^36 keys, so their chance to hit a right one is

(1.8*10^36)/(2^160) = 1.23 * 10^(-12)

which is one in a trillion. And this is only if they intentionally try to crack keys. If they just run normal vanity calculations, it doesn't matter how many keys they go over, as they'll eventually report just one key.

In the future (when the above figures may be slightly less ridicuolous), we might move on to longer key hashes.

So no, there's no risk.