use generate privates keys, since it is important key to access your account, you need strong combinations to make it
Which combinations are you talking about now?
If it is truly random, then your randomly generated private key could be 1 just as probably as anything else. I prefer to check that my randomly generated key is NOT very small or close to the biggest possible number.
That is a good exercise, however the bitcoin private keys would be pretty much uncrackable at least on modern computer. I think winning in a lottery has a bigger chance, than generating a private key that already has bitcoins on it.
What I understand is that generating private is not bad or not secure and what you need to put in check or always make sure is secure is your passcode/password which mean you have to use a genuine and uncrackable.
Generating vanity addresses is the same security as far as i know, as just using the automatically generated keys by your wallet. Also it does not matter if you have a password, unless you have a BIP39 encrypted key, which is not used by the wallets by standard. I use at least vanitygen for my keys, and generating a key on vanitygen is as safe as your computer is, assuming that you take care of your security well enough.