How to get from Burlington to Quebec City

I'm coming to Vermont this summer and then planned to go to Quebec City and then Montreal afterwards.

our plan is to rent a car in mid-July in New York and then use it to drive up to Vermont, where we'll be staying for a week in Middlebury

The question is how to get from Burlington, Vermont to Quebec City, where we will be staying for a week before flying home to Northern California.

according to my research, you can't rent a car in the States and return it to a Canadian location. also apparently the train is no longer running.

so how would you recommend getting from Burlington to Quebec City? We have a family of six so we'd prefer not to spend the money on flights if we can avoid it.