Doesn't matter much, anywhere behind the dryer from about 12" off the floor to anywhere above or below the height of the dryer, left or right.

The vent pipe is easy to accomodate to the location of the hole. Is it going directly through the wall to the outside ?

If you're planning a stacked washer/dryer it would make sense to place the vent line in the vicinity of the dryer vent port. If you don't know which washer dryer combination you'll be buying, then it might make sense to place the vent through the outside wall centered in the area of the laundry pair but slightly below the level of the bottom of the dryer. It's never a good idea to have vent ducting running vertically up, if it can be avoided. That being said, the shorter the vent line -- the better -- so if you can make your choice of appliances now, you can have the vent opening placed at an optimal location where it won't conflict with other connections for electrical, plumbing or gas lines -- but be as short as possible.

Can a stackable washer dryer (dryer on top) be vented through crawl space to outside?

Probably, but keep the length of the exhaust ducting as short and straight as possible.