Overview
My first ‘guide’… just because this method was not written in the walkthrough I read.***It may contain spoilers***
Getting the REPLANTING A LILY quest
When completing the THINNED BLOOD quest, you must free Lily from the hospital. After you do that Vandal will be really upset with you and will not want to sell you blood anymore.
The walkthrough I read (http://www.gamefaqs.com/pc/914819-vampire-the-masquerade-bloodlines/faqs/43148) from GameFAQS mentioned you could intimidate, pay, dementate/ dominate him, or complete the REPLANTING A LILY quest. This quest consists in replacing Lily´s place with someone else (If you have low humanity you may also be able to tell him a gruesome story).
Other possible Lilys
The walkthrough suggests using the girl on the first floor in the Asylum, or, if you´re a nosferatu, Patty from the Downtown club. HOWEVER I discovered that you are able to choose an easy pick from Santa Monica withought losing any Humanity points on the way.
I decided to wait until I met Patty to send her to vandal, but durring the side quest A BOUTY FOR THE HUNTER you meet a guy which you may send to Vandal instead. He´s name is GIMBLE, and when talking to him it is easy to get his creepy vibe. After sending him to Vandal you can still go downstairs to complete the A BOUTY FOR THE HUNTER quest and then return to Vandal to complete his quest too.
It´s done!
So, for those kind hearted vampires like me, that will not send a club girl to her demise (specialy when you can get blood from her anytime you want) but still want to get the extra XP that Vandal has to offer (and buy some blood of him too of course!), there is a way to do both.
I did complete this quest with the latest unofficial patch completely installed (9.0).
I hope this helps.