Thanks for the reply MICRO, but I'm suspicious about your information because it seems vague and lacking in detail. For example, you say that something is impossible but I think you mean infeasibly improbable.
Yes, that's what they meant. Note that for all practical purposes, you can consider that to be on the level of 'impossible'. I.e. say you roll a million dice - is it
possible that you'll roll a 4 on all million? Sure, it's possible. Now consider that the issue we're discussing here is several magnitudes more difficult still
Since I'm looking for details and proofs, I guess I find myself still wanting. Nonetheless, the point of your reply is noted: apparantly reducing the address space by a prefix is insignficant.
You would have to look for technical discussions about finding collisions for the private key in general. The fact that the address starts with a vanity doesn't actually 'reduce the search space'. I.e. you can't say 'generate only keys for which the address starts with this vanity'. The way it works is that it generates a whole lot of keys, calculates the address, and then checks if that address happens to start with the desired vanity.
If you
could do it the other way around, then 1SomeVan1ty is every bit as vulnerable as, say, 1x3pqDdtUza - a non-vanity (well, unless somebody considers that to be a vanity, of course) - and thus all Bitcoin addresses would be vulnerable.
In addition, while you can possibly get a collision on the address, you still can't spend from that address unless you have the correct private key. So even if you do happen to get a full match on the address, you might still not have the correct private key to go with it. (addresses are hashes of the public key, which have a smaller space than the public key, so at least there's a greater potential for a collision on the address)