Hi. Encrypted files are treated like any other file by the storage provider. But as pcloud only sees encrypted files, it will store these in the trash history, but without the vault structure. And you will not know where exactly these files should be stored in the vault structure. Means: restoring files from trash bin will not help you. You should consider your own history within your vault, if you are in the need to have one.
No limit.
There are User in this board who say that they are running vaults with several TB.
Be aware that this is not possible with the desktop app, as the app does not connect directly. If pcloud does provide a connection like WebDAV, you can connect this WebDAV to your file explorer and then create a vault in it. Please notice that some users in this board reported a terrible slow and unreliable connection to pcloud. I recommend using their client.