Upvote:2
As the other answer intimates, it would be possible to either do VWP but there is also a good chance you’ll get turned back at the Canadian border.
Look at it from the perspective of the US immigration officer. His or her job is to ascertain whether you might be trying to overstay your visa, whether you are working illegally, or whether you have immigration intent.
From all three perspectives , you seem like a risk:
All of the indicators are not good for you. You might re-enter without issue but you might also get turned back at the border and sent on a plane back to Germany from Canada if Canada also refuses to take you back. A double deportation/refusal doesn’t look good on a travel record.
Upvote:3
Yes. There is no rule preventing you from coming back on VWP after leaving on F-1, and given your vacation is several months after the end of your OPT, it seems reasonable to go home at the end of your OPT and come back on VWP for the vacation.
If you wanted to stay in the US until the vacation, you could apply for a Change of Status to B2, but it would be pretty unreasonable to stay in the US for months just for a short vacation at the end.