How to get this quest
See Poisoned Elixirs.
Following the investigation
You can continue your investigation either by talking to the elixir alchemist (if you know about Tofana's third theorem) or finding the document in the lair's secret room (if the vagrant told you about the secret door). The secret door is behind a bookshelf.
The lair is only open for one day. Failing to find the document within the day will fail the quest if you don't know about Tofana's third theorem.
Distracting Roland
You have two ways to distract Roland.
Confronting the alchemist
Talk again to the alchemist about the poisoned elixirs. You can:
Report to Roland to conclude the quest.