If the host is b.vimeocdn.com you can use https://secure-b.vimeocdn.com/ts/449/060/449060084_100.jpg to access the images over secure.
We are in the process of rolling out a new image system which will use the same host for secure and insecure. If the host of the image is i.vimeocdn.com then you can use https without changing the host.