If you go to the tour's web page, scroll down, and click "Planning Tips" https://www.ricksteves.com/tours/europe/my-way-alpine.
In that, there's this paragraph about flights:
There is more than one way to get to and from your tour. Most tour members will arrange to fly into Salzburg (Munich and Vienna are alternatives with easy train connections) and out of Geneva, using airport shuttle bus service from Chamonix ("multicity" on flight booking websites). Booking round-trip to one city will require a connecting flight, train trip, or additional overnight stay in Europe.
Note that while Vienna (ariport code VIE) is not too far from Salzburg, Venice (airport VCE) is not close. Unless you're spending time in Italy before the tour, you wouldn't want to fly to Venice.
I'd follow the recommendation and look on Kayak or Google Flights for a multi city flight. You have to check for all options, so look for:
1) Portland to Salzburg and Geneva to Portland
2) Portland to Munich and Geneva to Portland
3) Portland to Vienna and Geneva to Portland
4) San Francisco to Salzburg and Geneva to San Francisco
5) San Francisco to Munich and Geneva to San Francisco
6) San Francisco to Vienna and Geneva to San Francisco
Flying from the US west coast, most prefer to connect in Europe rather than in North America. It saves several hours, and you have more options if your first flight is delayed. For instance, if you fly San Francisco to New York to Vienna and your first segment is late, you have to wait a whole day to get to Vienna. If you fly San Francisco to Frankfurt to Vienna and your first segment is late, there will be more than one flight from Frankfurt to Vienna later that day.
From Portland (code PDX), the only nonstop to Europe is to Amsterdam, but from there you can connect easily to Vienna or Munich. From San Francisco (code SFO), there are multiple airlines with nonstops to various European cities, and Oakland (code OAK) now has flights to Europe on Norwegian. So, when looking for San Francisco, be sure to check the "look for nearby airports" box, or if OAK doesn't show up, search for it separately.
Munich has nonstops from SFO, on Lufthansa. Neither Vienna, Salzburg, nor Geneva have nonstops from SFO, OAK, or PDX, so you'll have to change; personally I'd avoid making more than one change.
There is no longer any rhyme or reason to airfares, so you just have to check all options and then figure out which one has the best combination of convenience and price for you. Do be sure to take EVERYTHING into consideration. For instance, if it costs $100 more to fly to Salzburg compared to Munich or Vienna, remember you're saving money as well as hassle, by not having to take a train on arrival.