Hello there,
Well iFastNet’s Ultimate Premium plan is a shared hosting plan meaning you’re sharing CPU, RAM and other related server resources with other websites while a Virtual Private Server (VPS) is like a dedicated server but not really since you won’t own the whole server unlike a dedicated server, with VPS you are provided with more access with the servers like you have SSH access and for the server resources, I think you’ll have your own small partition that is dedicated for your site only.
Well Shared Hosting is good enough for Blogs, Static Sites, Simple Forums and VPS is good when your site is starting to get many traffic and uses too much server resources.