It was pretty easy for me to do this as a Nosferatu. The easiest way to do this is to fully upgrade Obfuscate to level 5. In this case, you will need to stealthily kill the guard who walks in the backyard and drain the guard who is standing at the back door, then you will not raise the alarm and enter the mansion without noise. In case you haven't maxed out Obfuscate that much, you can use a level 3 or 4 animalism and stealthily kill the guards in the backyard, just be sure to turn on Obfuscate of any level before doing so. Remember that from Obfuscate you can apply your disciplines while still remaining invisible.