This guide explains how to open the basement vault in the Boyle Estate. It does require you to have cleared the top area, it shouldn’t be too difficult as the guards patrol around the place alone for the most part.
There is a video version of this guide at the end of this guide.
The basement vault key is on the smaller of the two upstairs areas. There are two main rooms in that area, one of which is Lady Boyle’s room:
Inside you will find the basement vault key on her writing table:
The vault is in the wine cellar with the boat in it. It is in the far corner in the upper region of the basement (higher than the boat).
You can get around all of this by possessing a rat and going into the basement vault that way. It is in the kitchen area before the basement: