Maybe I am wrong, but economically speaking you are better to use as much of Amtrak instead of VIA - even if you travel a greater distance. Amtrak has often great deals, while the deals on VIA are less frequent.
It is worth to check out the Amtrak rail sale or other opportunities.
The current "normal" fare I do see from St. Louis to New York is $143 if you use trains #422 from St. Louis to Chicago and the train #48 from Chicago to NY.
The New York - Montreal fare I do see is $68 on the train #69.
You may get lucky if you look at the rail sale
http://tickets.amtrak.com/Amtrak/railsale
What I do see now is that there is a sympathetic fare of just $33.20 between CHI and BOS if you go by train#448 or just $33.00 between CHI and NY if you go by train #48.
So I would try following:
1) buy the regular St. Louis -> Chicago on the train#322. I would try to use the 25% off promo (code H30). This yields me a price of $15.75. The train departs St. Louis 8:45 am and arrives Chicago 2:25 pm
2) buy the Chicago -> Albany on the train #448 via Amtrak Rail Sale (it is $30.80). The train departs Chicago at 7:00 pm and arrives Albany at 11:00 am next day. (This train seem to be difficult to get the sale tough). The train #48 has the same pricing but arrives 10 minutes earlier. I would never count on a 10 minute connection, but you MAY get lucky. (5% chance) - see below.
3) The Albany -> Montreal is only $45, but the train#69 departs at 11:00 am which is a definitly BAD connection, so you could land up waiting an entire day!
As you see, the most critical and expensive part of the travel would be the big leg from CHI, while the one from St. Louis to CHI or from ALB (NYC) to Montreal seem to be easy. Depending on your time you can get more creative and try a CHI -> Pittsburgh->NYC connection. The CHI -> Pittsburgh is now only $22.00 via "rail sale" and the Pittsurgh -> NYC shouldn't be a problem.
Therefore the lowest fare I did discover so far is $15.75+$30.80+$45 = $91.55.
If you would use the H305 promo on the Albany -> Montreal portion, you can get that portion further down to $30.75 which would mean the absolute minimum possible fare would be today $77.3 for the entire trip.
This couls of course change if you would see a NYC -> Montreal "promo" coming up next weeks or an other good promo from CHI eastwards. I would also watch the CHI - WAS price and snap it if it goes low.
Good luck!
