If you can get a letter from VW Canada on their letterhead that states that your car (identified by the VIN) is in compliance with US DOT and US EPA regulations at the time of manufacture, then provide that to the buyer, you should be just fine. If you are exceedingly lucky and have official VWAG stickers on the radiator support (such as the ones with the vacuum line diagram) that state that compliance already, then you are golden and might not even need the letter.
The buyer should be able to check that with US Customs to verify that such a document is sufficient to allow it to be imported.
The buyer will not be able to obtain such a letter if he buys the car and tries to get the letter before crossing the border, so having you as the current owner do it is about the only way of getting it done so the car isn't impounded at the border. Or maybe impounded is not the right verb - the car would not be allowed into the US until those DOT and EPA requirements are satisfied.
I have no idea how long any of this would take.
Good luck, and cheers!
PH