There could be weaknesses introduced by the mining alg that make it predictable but, naively, it's the probability that the rest of the bits randomly selected are the same.

So, assuming your vanity is 6 bytes, thats 32-6 => 26 bytes or 208 bits of entropy if i'm remembering the pubkey size right, so chances are 1/2^208 => 2.4308653429×10⁻⁶³ to choose the same key that was mined, then doing that over and over until you find it. 208 bits is pretty safe. #[2]