I have that exact plan for March except I won't take the Builder, I'll take a Lyft to Everett Amtrak to catch the Cascades.
Here is the current situation and my plans:
1. As of right now, 9/14, there is no Amtrak Cascades or Thruway service to Vancouver. There is no announced date for resumption, but the Cascades to Vancouver show in the reservation system starting January 2022. It is generally expected that service will resume fairly shortly after the border opens to the vaccinated both directions, and Washington DOT appears to support that. I have a reservation for the morning Cascades train 516 to Vancouver on 3/25.
2. Primarily due to the nebulous quarantine plan that Canada requires, I am planning to spend 3 days in Vancouver so I have a "quarantine plan" especially in the event I am selected for the random COVID test right there at the border. Otherwise, my "plan" will be to stay in the hotel if I get sick rather than getting on the train. My own personal estimation of the situation is that planning to get straight on the train same day as arriving in Canada would raise issues on how much of a "quarantine plan" that is. Normally I catch the Canadian that same afternoon.
3. As of now you must have proof of both vaccinations completed at least 14 days before presenting yourself for entry. The CDC card is accepted as proof. You must also have negative results from a COVID PCR (not antigen) test administered within 72 hours of arriving at the border. Right now you can get a PCR test for free from the King County Health Department at various locations in and around Seattle with pretty quick results for free. You don't have to be a resident or have proof of residency. I am planning to go down there and get one a day or two before I leave, depending on how quickly results are being returned at that time.
4. You must upload a copy of your proof of vaccination, your PCR test results, and describe your quarantine "plan" on the ArriveCan app or website before arriving at the border. I plan on doing it as soon as my PCR results come back.
5. Canadian Border Services may randomly select you for another COVID test during Immigration processing in addition to your pre-arrival one. I have read in unofficial sources that you may have to be prepared to quarantine until results are received from that test, which could be up to 3 days. Hence that 3 day layover I built into Vancouver.
5. Returning to the US at a land border. Right now you don't need a test at land borders to re enter the US. I am not sure what the situation will be once they more fully reopen to citizens of other countries. It may well change. Entry by air requires a negative COVID test ( antigen is ok for US entry). Since I don't know what the US rules will be, I have left my return border crossing plans open to allow for time to get tested and receive results should that become required. I'll firm it up when the US reopens more fully and that becomes clear.
It will still be a pain if current protocols remain in place, which I think is more likely than not. But it manageable in a way the Detroit-Windsor crossing isn't. It does, of course, depend on the Cascades or at least the Thruway buses running, but I think that is a pretty good bet.
As I mentioned before, I will be driving to Canada twice before my March VIA trip, so will have a much better feel for the process by then. And hopefully that will give me a better understanding of their interpretation of that quarantine plan requirement.
Finally CBSA is every bit as tough and hard nosed as US CBP. Make sure and dot your i's and cross your t's. They don't hesitate to turn people who don't around.