Installing Vincent the Vicar on your device should be a piece of cake. This is especially the case if you purchase it using the device that you wish to use to read it. It should then download with delightful ease onto your reader.
Installing Vincent the Vicar is easiest if you buy it using the device on which you intend to read it. In this case just buy, download and Bob's your uncle.
The simplest alternative method is to email the file to yourself and open the email on the device you want to read on. Click on the file and you will be able to open it in the reader on your device.
Remember: The .mobi file is for Kindles (and the Kindle app on iPads and iPhones) and the .epub file is for Apple and other devices.
The other option is to download Vincent the Vicar onto your computer and then side-load the file onto your device. You probably know how to do this, but if not here are some detailed instructions. You can also email the file to yourself and open the email on the device you want to read on. Click on the file and you will be able to open it in the reader on your device.
In very brief summary:
Kindle: connect your Kindle to your computer, click on it until you are in the documents folder and then drag and drop Vincent the Vicar (.mobi) into that documents folder.
Use the Apple iCloud system. Put the .epub file in your iCloud drive on your computer. Open your iCloud drive on your iOS divice and Vincent should be there. Select the file, select the share icon in the top corner, select Copy to IBooks.