Do vanity addresses somehow decrease the security of my private key?
No they do not.
A vanity adress is only trying to pattern match a specific part of an address. The complexity remains the same.
To generate the vanity address it usually takes millions of attempts per second to find a public key which matches your pattern. This is done by cycling through private keys and and checking the corresponding public key.
If an attacked was to do the same thing then they would still need to cycle through every combination of your pattern aswell, which is just as extensive.
They do if you reuse the address multiple times. When you spend coins sent to an address your public key is exposed to the network. This reduces security slightly.
Address reuse also leads to a loss of privacy. Remember all transactions are public information. By reusing the same address over and over again you are letting the world know how many coins you are receiving and sending!