The full private key for the vanity address can be obtained using the partial private key of the third party and the partial private key of service, that generated the vanity address.
You are using a VERY strange and unusual definition for the word "safe".
As I understand it, you are suggesting...
A complete stranger that I know nothing about and have no reason to trust generates a partial private key for himself and gives me a copy of it.
Another complete stranger that I know nothing about and have no reason to trust uses the partial public key that is associated with that partial private key to generate a vanity address.
This second stranger has the second part of the private key for himself and gives me a copy of it.
So, now two complete strangers that I know nothing about and have no reason to trust each have a different half of my private key, and I'm supposed to feel "safe" about that?
Furthermore, the first stranger will be sharing his half of my private key with lots of other people (so they can all get vanity addresses generated with the same effort by the second stranger).
Now, lots of people all have one half of my private key, and the vanity address generator has the other half. Furthermore, depending on the method used by the vanity address generator, it may be possible for someone else to use their vanity address partial private key from the same generator to calculate mine?
No thanks.