Prevent image resize

Hello! The site I’m working on uses vfb (pro) to allow users to upload images. I have already set a max upload size, but by default wordpress creates multiple sizes in /uploads/vfb. I would like to prevent this and only keep one version of the file, i.e. the one that was uploaded. I’d like this behaviour only for images uploaded through vfb, not for the ones added by content editors in posts (I know that vfb attachments are one kind of post, as far as the db is concerned).
Any idea?
Also, when limiting the allowed file extensions in a comma-separated list, do I write them as “.jpg,.png…” or without the .dot? Stupid question, I know…