It only uses memory when the VM is running, as soon as you shut it down (or pause* it) it frees up that memory.

It will continue to use the disk space though, otherwise you'd need to install Vista etc on it every time you used it. By picking a dynamic VHD you minimise the space it uses.

For example if you ask it to create a 50GB dynamic VHD, initially the disk space used will be only a few MBs at most. As you write to the VHD (by installing Vista on to it) the dynamic VHD will grow to store the data written (but to the copy of Vista running within the VM the disk will always appear to be 50GB in size).

So if say your Vista install takes 15GB, that is how much space your dynamically sized VHD will consume.

* if you pause the VM, rather than shutting Vista down, it will use a bit of extra disk space while paused, as VBox saves the active contents of the RAM of the VM to disk when you pause it (with the benefit that when you start the VM again it starts up in the same state when you paused it, just like if you had hibernated from within Windows on a physical machine)