As you are staying less than 3 months in North America, there will be no problem.
When you enter the US the first time, you'll have to fill a small green paper, the I-94. This form should be given back when you leave the US, and it is a proof you actually left US territory.
So you will not give it back when you go to Vancouver, but when you leave Seattle before going back to Germany. When you cross the terrestrial border to enter the US (on the way to Seattle), showing this green paper (and the entry stamp) should be enough, and you will go through customs without troubles.
Remember, the reason why this works is that between your first entry and the time you leave the US at the end of the trip, there is less than 3 months.
EDIT : apparently the procedure changed in this precise case, the I94 is no more filled when arriving by the air/sea. To be confirmed by someone who travelled recently, as some resources claim the opposite and no document is dated.