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.
quoting nevent1q…9wgpGood question! I am not sure on the chances of someone else mining a duplicate key. I imagine pretty slim even with “shorter” vanity words or phrases but I don’t know what the cutoff is as far as number of characters goes. I imagine for longer characters though and good key management you would be pretty safe.