Re-enter USA from Canada with Visa Waiver Program

There won't be any issue, just maybe a bit of a hassle. The Visa Waiver Program functions differently for air and land travel since the introduction of ESTA. Since you plan to do both, you will need to go through both procedures.

1. Arriving by plane: At least two weeks before your journey go to the ESTA homepage and fill out the electronic form. Note that you'll need to pay for that by credit card ($14), but you should have one, anyway, for travelling to the US. The ESTA registration will be valid for 2 years (which means you don't have to do it every time you go to the US), but of course, you'll still be subject to the normal rules for a visitor visa, which means you have to leave the country within 3 months of your last entry stamp in your passport. You will not be filling out a green I-94 on the plane.

2. Arrival by car: You will have to wait (so choose a small border crossing - you'll be made to wait a bit even if there's no line) to fill out the green I-94. You'll have to pay for it ($6/person). In your case, you'll hand the I-94-stub back to the customs officer at the border; in case you were flying out of the US, you would have had to make sure to hand the I-94-stub to a airplane representative at check-in or at the gate before leaving the country.

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.

Just had a chat with a US immigration officer at a Canadian land crossing. She confirmed that you do not surrender the I-94 until you have finished visiting, providing the visits are within the 3 month period of the I-94. She also confirmed that Canadian border officials cannot remove the I-94 from your passport, you must offer it. I have found that they rarely offer to take it from you. The onus is on you to request they take it.