Are vanity addresses Vanitygen generated really safe to use?
The key point here is that vanitygen is not deterministic - It relies on entropy provided by the system it is running on, which is why running it multiple times with the same pattern will produce different addresses.

As long as the entropy provided by the system used to generate the address is sufficiently random, keys produced by vanity gen are as good as keys produced by any other program that would have used the same entropy.

Naturally, you should be verifying that the binary you are running matches the source code, and is using a good source of randomness, before moving any funds to those addresses.