Entry to Estonia?

I haven't visited the city of Frankfurt, but the other places you list have a lot to recommend them. You could also fly to Stockholm (more interesting than Helsinki) or Helsinki (worth a visit but not in Stockholm's league to me) and take a ferry to Tallinn. The ferry from Stockholm runs overnight, so you don't lose much sightseeing time, and there's a fabulous view of the Stockholm Archipelago in the first few hours after departure.

You haven't told us your starting point in Canada, so we cannot check flight schedules to see what flight itinerary might be practical for you. I'd look at flight times and fares first, then choose my pre-trip stop from among the options that looked acceptable. The Wikipedia entry for the Tallinn airport lists destinations with direct flights. Many of the flights are on budget airlines, so you have to pay attention to extra charges for seat selection as well as luggage. When you factor budget flights into the equation, your universe of practical stops expands greatly. When I'm looking for intra-European flights, I consult skyscanner.com; for transatlantic flights I use Google Flights.

https://en.wikipedia.org/wiki/Tallinn_Airport

I imagine you are aware that a multi-city airline ticket would allow you to book a ticket into your choice of pre-trip city and out of Vilnius, as long as you can find an alliance with at least one airline serving Tallinn and one serving Vilnius. I wouldn't want to start the trip back to Canada with a separately-ticketed short hop on a budget airline; a problem with that flight could cause you to miss the transatlantic flight, requiring purchase of a very costly last-minute, one-way ticket home.

Thank you for the reminder about cheap short flights!
I think we will go from Vilnius to our “port” in Europe the day before our long flight home (YUL)

It appears that our cheapest option is Air Transat YUL to CDG, with Air Baltic links to Tallian and back from Vilnius.

BUT…what about a London entry into Europe? Does anyone have experience flying from London to Tallinn?

Stockholm would be a great choice. When you are ready to travel to Tallinn, you can take the overnight ferry. The Stockholm archipelago is beautiful and the ferry including a cabin usually costs about the same as a hotel night.

Otherwise, just pick a city and see if it has nonstop flights to Tallinn. If you want to get an idea of which airlines/cities connect with Tallinn, look at the airport wiki page https://en.wikipedia.org/wiki/Tallinn_Airport

There are nonstop flights from London to Tallinn but not everyday.

Your choice will be Ryanair, Wizzair or Air Baltic.

Use Google Flights to see the schedules.

Remember, the UK is not part of the EU or Schengen.

Next fall I am starting a tour in Tallinn, but am arriving early in Helsinki. I made this choice because Finnair had a nonstop from Seattle that I could use frequent flier miles on at a very reasonable redemption. I don’t know if they fly from your home airport, but it is worth checking out. After a few days in Helsinki, I plan to take the ferry across to Tallinn

A London entry has the drawback that if you plan to also stay there, you need to clear immigration twice - the UK is neither EU nor Schengen anymore. Just a hint…

Interestingly, I have flown both Frankfurt-Tallinn and Frankfurt-Montreal this year, and both worked nicely. But I digress.

I am fairly confident that you will find nice things to do in right about any major European city that connects directly to Tallinn - so it only comes down to what is most convenient, cheap and interesting to you.

Tallinn itself is surprisingly similar to Quebec City btw - a small, walled historical old town and the bulk of the city outside of the walls.

From YUL (Montreal Trudeau) the only non stop flights to London arrive at Heathrow. You would then have to self connect on separate tickets to Gatwick or Luton, preferably allowing a night in London to cover for flight irregularities.
So London is not a particularly desirable connection point in my eyes.
I checked for non stop flights from YUL to Edinburgh or Manchester for same airport connections but there don't seem to be any.
Dublin is possible - seasonal AC flights then self connect to Ryanair.

Since you're looking for something to adjust your body clock and rest, I wouldn't choose any place that was too far away. Jet laand sleep deprivation can be difficult to deal with.

Honestly, i agree with the others about Stockholm. It's a beautiful city, it would be easy to get to and there is a lot to see and do. And it's quick and easy to get to Tallinn from there.

I just booked flights to VNO connecting in Copenhagen with a return from TLL connecting in Amsterdam (Air Baltic). Amsterdam is an easily visited English speaking city with trams from the airport.

You could a fly into Helsinki for a visit and then take the 2 hour ferry to Tallinn. If you don't have much time in Tallinn, you could also just go directly there and enjoy a longer visit before you busy tour schedule.

Have a great trip!
Anne