depart from Seattle or Vancouver?

In which country do you live? While it is impossible to know the situation in four months, it might be a good idea to reduce the number of border crossings in the current situation.

If you are a US citizen, you need to consider the possibility that you may not be allowed to enter Canada in May. What if Canada closes their border again?

Greece's entry restrictions are not likely to be different whether you enter from Canada or the US. I suggest a Google search for terms like "Covid entry rules for Greece" for the specifics. Most European countries are now requiring a recent test as well as vaccination. Whether flying to Athens from Seattle or Vancouver, you'll probably need to connect at a European hub like London, Paris, Amsterdam, or Frankfurt. Those are the ones with nonstops from Seattle, there may be others from Vancouver.

To minimize Covid rule complications, you could look for a nonstop service between Athens and a US city, probably on the east coast, with a connection to Seattle. That way Greece and the US would be the only countries whose rules you'd need to be concerned about. And, of course, no one knows what the situation will be in May.

From the wording of their post, I believe they are US residents.

How much cheaper is the Vancouver flight, and what airlines are involved? Where do you change planes on the way? Also think about the details and logistics of getting to each airport. In particular, how would you get to the Vancouver and the airport—-train, bus, car?

We flew home by way of Vancouver one year, when I could not get a British Airways business class reward flight to Seattle, but could to Vancouver. We ended up spending the night in a hotel there and taking the train home the next day. It was fun, but added extra time and expense to the trip.

We had a flight to Europe booked from Vancouver in March, 2020 (we did not go, of course). I decided to avoid the overnight there in the city before the flight, and the train and bus schedules did not mesh well with the flight time. So I found that we could rent a car one-way from Avis for $80, and booked that. All we had to do was get ourselves to the Seattle downtown rental office, and then drive, allowing extra time for the border crossing. But since we did not go, I can’t say how it all worked out.

https://travel.gc.ca/travel-covid/travel-restrictions/domestic-travel

If you’re 12 years of age plus 4 months, or older, you’ll need to be fully vaccinated in order to board:

domestic or international flights departing from most airports in Canada, including charter and foreign airlines carrying commercial passengers
Medevac flights are excluded, regardless of where they depart or land
Private flights that don't require access to airports with a vaccination requirement are excluded
VIA Rail and Rocky Mountaineer trains
Contact your airline or railway company about their boarding requirements and refund policies.

Airports with vaccination requirement
Find out if you're ready to board
To board your flight or train, if you’re 12 years of age plus 4 months, or older, you must:

be fully vaccinated with accepted vaccines
be ready at any point in your journey to show your official proof of vaccination
have received your second dose at least 14 full days prior to the day you travel
have no signs or symptoms of COVID-19
wear a mask
Accepted vaccines
Connecting flights and rail after entering Canada
Foreign nationals departing Canada
COVID-19 tests: if you qualify as fully vaccinated, a COVID-19 test is not needed to board a flight or train in Canada unless you’re flying to a country that requires it.

If you fail to provide proof of vaccination (or valid COVID-19 test result if you have an exemption from your carrier), you won’t be allowed to travel and could face penalties or fines.

Vaccination is required for travel within and to depart Canada. A valid COVID-19 molecular test will no longer be accepted as an alternative to vaccination unless you’re eligible for one of the limited exemptions.

Wow! Thank you all for your replies. This is very helpful. We have never traveled with a tour and did not think of some of these things. We have done a fair amount of pre-Covid travel on our own but have not traveled since 2018. So many additional things to think about these days. The possibility that border crossings could change at any time certainly compounds potential problems. To quickly address the questions raised: We live in the US; both Seattle and Vancouver are about 3 hours away; the cost difference for the two of us is about $700 total. Taking in what you have all said, we will go with Seattle.

Thank you all again. I am sure I will have more questions as departure times get closer. We are really looking forward to this new adventure.

I think you’re making a good choice.

We faced the same dilemma when booking tickets for our trip to Spain. Montreal is close and convenient (pre-covid) and would have saved us thousands for our 10-person group. We decided to spend the extra and fly out of the US; we thought it too risky in light of the reality of ever-changing border requirements. And had we booked our tickets out of Montreal and needed to change them last minute, the cost would be much, much higher than the initial difference.

We were very happy with our choice, and it was one less thing to stress about.