Would it be possible to attatch the forgotten vale to skyrims worldspace, so I could travel there without going through the cave?

You can fast travel there once you've found it, just like any other location.

Do you just want an alternative entrance route, so you never need to use the cave the first time? That would break the quest, of course, as there are things you need to do in the cave, before everything in the Vale will work properly.

I just want to be able to walk over the mountains into the vale. Not all my characters do Dawnguard but would like to explore the vale.

Then just use the console to "find" it. Use coc to go there, and then back. Once the map marker appears, you've got access.

The wayshrines probably won't do anything. Without the quest running, the guardians won't open them. The Sanctum will remain locked. No idea if the dragons will be active.

Unfortunately I don't think such a mod exists. Best options are probably the ones ghastley gave you

Also I'm pretty sure the Vale worldspace is physically larger than the area the game implies it's located. So I don't think it would even fit if someone actually found a workable way to move all of that to the Tamriel worldsdpace.

Check out this mod: http://www.nexusmods.com/skyrim/mods/14976/?