When do you consider to move to VPS hosting or you prefer to stay with shared host that says it offers plenty of monthly transfer and disc space?

You may consider moving to a VPS when your site is generating a lot of revenue and it’s using a lot of data transfer. When you have a VPS you have real bandwidth and space.

A VPS can still be a decent way to avoid spending a lot of money on a dedicated server, while usage is still moderate, but a bit too much for a regular shared hosting environment.

I guess small sites are probably better off with shared hosting.

if your website is so big that it needs more resources to utilize, you require root access, receive a lot of traffic, run different applications/scripts then you should opt for VPS.