I have purchased vehicle in the USA and brought them into Canada. So we're going the other way but it may be similar. I needed to fax a copy of the title to US customs 72 hrs in advance to export it. Not all vehicles are able to be imported but there is a very detailed list of can and can'ts. If it is older you should be good but check it out. I picked up a toyota pickup in Ohio and was crossing in Detroit driving it. When I got to customs the dickhead agent there tried telling me I only faxed the title 65 hrs ago. I definitely faxed it 4 business days earlier so he was full of shit and on a power trip(my dad lined up in the wrong line and this guy is yelling at him and has his hand on his gun, like wtf?) so after sitting at customs for a couple hrs and them not letting us proceed with the export we driven into Canada anyways. Canada customs couldn't have cared less if we exported it from the US before we imported. Because my truck was more than 25 years old I didn't require to go through the imported vehicle inspection which costs $200 up front at time of import and can have some very strict requirements. We took the truck back to US customs the next day and exported it from the US.
I had purchased a 30 day temporary permit in Ohio to drive it home and got pulled over about 28 days later in Ontario and I thoughts I was in trouble but the officer asked me if it was legal to drive with the Ohio temp and I said yup. He then proceeded to ask me how to import a vehicle because he was planning to do the same. Cool.
My advice to you is that US customs is going to want to know about this van getting imported into the country. I would call them directly and ask. (Good luck, they don't like answering questions. They will just tell you to go to the website. Just call back and ask again till you get somebody who isn't a d-bag.)
I would not import a vehicle again unless it was something unavailable or rare in Canada or it was a screaming deal.