>>368215472
Or more strictly; VRAM is paged and it uses an algorithm such as 'last used' to determine what to swap out with RAM. It's not 'garbage collected' like I said. As demands for rendering are given to the GPU it will load resources from RAM when needed, replacing whatever hasn't been used in the longest time (or some other very quick 'how do I determine which page to replace' algorithm)
However, having a very large texture rendering eats VRAM like nothing else and it can't be swapped out because it's in use.