My idea of the Vale, and the reason why it's considered impregnable without dragons (or maybe boats), is because of the bottleneck in the Bloody Gate; however, looking at a map of the area, it seems that it would be easy to bypass that bottleneck by entering the Vale through The Bite. It would be a longer walk to The Eyrie, but considering it wouldn't be that much longer, it looks like an obvious choice for an attack.
I'm assuming there's more to this than what I know from watching the show and looking at a map, so my question is: are there plausible reasons that would make The Bite a less preferable entry point than it looks at face value?