Going to copy what I wrote to John to this thread as well, for anyone who is interested in the future:

What time are you thinking of heading out, and what are you riding?

Are you going via St Petersburg, or skipping the Russian visa and going straight up?

From Vilnius to Tallinn you've got basically two routes, westerly and easterly. The westerly is the Vilnius-Panevezys-Riga-Pärnu-Tallinn one, fairly urban. Starts as autobahn out of Vilnius, freeway most of the way to Riga, traffic can get quite bad near there. Unless you're stopping over for a visit in Riga, you can take the bypass around the city and save some time and effort. Past Pärnu you have the chance the detour over to the Estonian islands.

Easterly is more rural and IMO more scenic, you go via Utena and detouring around Daugavpils, crossing at either Valga or Ape/Mooste and then via Tartu where I am based. From here you can either take the highway to Tallinn, or a couple of more scenic options, or just go straight up north to Narva or Rakvere, there's definitely scenery there too.

In general, Vilnius to Tallinn can be done in one long day's riding. In Latvia have a look at Cesis/Sigulda-Turaida, nice scenic areas on the road between Riga and Pärnu.

In Estonia, the national forestry service maintains campsites, forest huts, etc. that you can usually use for free. Info here: https://www.loodusegakoos.ee/where-to-go/search-options

Plenty of ferries between Tallinn and Helsinki, the big one is Tallink, but Viking Line can be a bit cheaper. Eckerö is the third one. If you get a Louis.eu loyal customer card, you can get your bike shipped for free with a deck ticket on Tallink, but you have to call their customer support team and book that way, the discount does not work online. There also used to a be a passenger-only hydrofoil, that appears to have gone bankrupt for good.

Viking Line arrives right in downtown Helsinki, Tallink and Eckerö arrive a little ways off, but there's no real difference in terms of getting onto freeways quickly.

Heading north, again you have a few options. Finland is big, expensive, and mostly flat, there isn't that much historical architecture, so I tell people to at least consider hopping on the car train (http://www.vr.fi). You can put your bike in the car-carrier and have your own room with a bed and a shower. Train leaves around 7pm, arrives in Lapland around 8am the next morning. Cost for me with the bike and a private ensuite was ~250 euros a couple years ago, and otherwise it's a two-day kinda boring ride, and you will spend as much money on gas and hotels. Trains terminate either in Rovaniemi or in Kolari, depending on the day of the week.

Wild camping is legal in Finland and Norway AFAIK, but any kind of hotel will be expensive. The cheaper option is an organized campsite with facilities, or a mökk (hytte in Norway/Sweden) - a little wooden wilderness hut. Might have a kitchenette, unlikely to have its own shower, sometimes won't even have bedlinen, but it'll be dry and fairly warm. Plenty of those all over the road, look for signs.

Riding up, again you have basically two paths, west or east. West is via Oulu and then along the Swedish border via Muonio/Kolari, turning off towards Enontekiö. East is via Rovaniemi/Ivalo, turning off at Kaamanen. I think the Western path is a bit better, but there's not much difference between them really.

Be VERY afraid of reindeer in the road! In the Arctic they graze freely, are absolutely fearless, and WILL be standing in the middle of the road behind a blind crest. Not a question of "if" or "when", you WILL have that experience on your ride to Nordkapp. Watch for drivers coming the other way flashing their lights to warn you.

In Nordkapp it's either camping or really expensive hotels. Even the hostel in Honnigsvag is expensive. But the roads are wonderful.

On the way back, definitely more scenic to go via Norway. Sweden is fast and boring. I'd go via Tromsö and Senja island to Andenes, then ride all the way down Lofoten to Moskenes and catch the fast ferry to Bodo.

Stay off the E6 in Norway - it's a clogged arterial road full of trucks, usually one way in each direction. Stick to the Rv-roads and the National Tourist Roads: https://www.nasjonaleturistveger.no/en

South of Trondheim, you can get to Kristiansund (boring nothing town) and take the expensive tunnel out to the Atlantic Road - tbh by that time you will have seen enough of the Norwegian coastline to be underwhelmed, but it's a bucket list thing with that humpback bridge. Then head inland to the Trollstigen (best done north-to-south) and Geirangerfjord. Then depending on where you're terminating your trip, you can head south to Oslo/Kristiansand and the ferries to Denmark, or east to Stockholm and the Baltic ferries (alternatively as far south as Karlshamn for the ferry to Klaipeda, which sucks as a city, but is right next to the Curonian spit, and that's worth a visit).

Hope that helps.