In order for these services to be trusted, they must have an appropriate level of security, which should not allow leaks of public and private keys. It is not in their interests to earn long trust, and then lose it because of data leakage.
In addition, issued private keys do not need to be stored and can be destroyed.
The point is that they have to be trusted. But this whole thing can be done in a more trustless manner that does not require a third party at all, just the participants and you. So why do you have to introduce a trusted third party into this? The trusted third party is single point of failure; it does not require them to be malicious, it just requires them to do something stupid, and people tend to do stupid things accidentally.
No, only you can get the full private key of your vanity address. And with an insignificant amount of probability, the full private key of your vanity address can get by "strangers" if they enter into collusion, but they both lose trust from other users.
Then we have to trust that they don't collude. Why should we be trusting them to not collude? We don't know who they are, we have never met them, we don't know what they are doing behind closed doors. What reason should we ever trust these strangers with our partial private keys?
P.S. Don't make consecutive posts replying to people like you just did. Instead make one big post with your replies to the people you want to reply to.