Based on the Air Canada link provided above, I'm inclined to agree with californiadreamin' that a Visa will not be required (bolding mine).
In accordance with the Canadian government’s Transit without Visa (TWOV) program, citizens of certain Asian countries who are transiting through Vancouver International or Toronto Pearson International airports to or from the US are not required to obtain a Canadian temporary resident visa, provided:
•They are travelling to Canada on an Air Canada or Air Canada Express (operated by Jazz only) flight,
•They hold a valid passport from their country,
•They hold a US visa that is valid upon their arrival in the US.
The TWOV program applies to the following passengers only:
•Citizens from Indonesia, the Philippines, Taiwan (Chinese Taipei) and Thailand
•Citizens of China who are travelling from Beijing, Hong Kong or Shanghai.
From what I can tell...
- Thai citizens are eligible for the program.
- Flights from the US are eligible. Not only does the AC website specifically state this, but why else would they state that Air Canada Express flights qualify? AFAIK Jazz doesn't fly to Asia.
- There is nothing stating that citizens from Indonesia, the Phillipines, Taiwan and Thailand have to be travelling to/from those countries.
Of course, I would recommend that the OP check with a Canadian consulate to confirm the above.
The citizenship and immigration Canada website linked to only refers to what airports and airlines need to do in order to be eligible to qualify for the program and doesn't really affect the OP as he will be transiting an airport and using an airline that do qualify.