If you edit an image using the ImageVault editor in EPiServer, then you will only get a copy of the original ImageVault image. It will not affect the original image or any other property in EPiServer that uses the same image.

So the only way to "reuse" an ImageVault image witiin EPiServer is to use a block?

That is correct. A media item is often context sensitive and it is better to create a structure of reusable context blocks that combines images, texts (aso) and reuse these if you need reusability.

If you, from two different pages, inserts the same image with the same settings (width/height/crop) you will reuse the same url. If you then change the image on one of the pages (change width/height/crop or change to another one completely) it will not affect the image other page.