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?
I understood you. But look at the online wallets. They own the full private keys of your addresses. Such an online wallet at any time can steal your coins and he does not have to collude with anyone. You have to completely trust them, but in return you get the convenience to make transactions with the wallet.
For vanity addresses, the situation is similar - you trust the service for generating vanity addresses and the service for providing public keys, and in return you get a vanity address at a significantly low price. Even with vanity addresses, security is much higher, because to steal your coins
simultaneously 2 services should be dishonest, they must find each other, agree among themselves who will get the full private key and steal coins and how they will be shared, and most importantly - they both will lose the trust of users and its business.
In addition, if one of the services turns out to be honest, it can warn other users in advance that the other service is dishonest.