Question about an international flight booking?

>>Do I need to go the whole way with the scheduled flight to be able to fly home with them?

Yes.

If it costs $300 to change your return to be from Vienna instead of St P and Moscow to Vienna only costs $150 then your cost to visit Moscow is $150 plus whatever cost to get there from St P.

You could buy a flight from Moscow to St P to connect to your flight to Vienna but there would be a risk in missing the connection. Allow at least 4 hours to connect and have one or two backup flights - flights that would get you to St P at least 2 hours before your Vienna flight.

Or, maybe better, return to St P and stay there over at least one night before your flight to Vienna.

The point, in all cases, is that if you use your existing ticket then you don't want to miss the flight from St P to Vienna.

I found Moscow to be more interesting that I expected and consider it to be worth the extra $150, if not the $300. It is worth visiting even if it means going back to St P.

IF you miss ANY leg of your flight, then the <<entire>> remainder of your itinerary is canceled. There will be no refund and no credit. You will end up having to buy new, last minute, expensive tickets to get home.

Airlines do this <<precisely>> to stop people from buying ticket from A to C that go through B. The cost from A to C is often lower than the cost from B to C. They don't want to have an empty seat from A to B, then have you get on at point B to continue.

You may not have had the exact plan in mind, but it amounts to the same thing.

So - either call the airline and make the ticket change, or catch the flight on the original itinerary. The choice is up to you.

Also, it make no difference if this is an international flight or a domestic flight. The airline will still cancel everything after you are a "no show" for one of the flight segments.

if you miss the first leg of the flight the rest of your reservation will be cancelled