[colanderman]

If you've found the private key for myveryveryveryverylongvanitypublickey, then so can an adversary.

Perhaps they already have reversed it because they guessed it might be desirable. Or maybe it has numeric properties which make it specifically easy to reverse (perhaps why you were able to discover it yourself).

(Also note that selecting the entire key to be vanity doesn't reduce the entropy to 0, just to the entropy of the vanity phrase. So a full vanity key may be equivalent to something like a 32-bit random public key.)