Welcome to Cruise Chat. Generally round trip tickets are better. With that said, it seems you can't really go by any set rule with the airlines. Fares vary from airline to airline, and the competition you have from the destination your flying out of and going to. There's no rhyme or reason to what the airline do anymore. Here's a good example I just did some checks using the same airline and dates of Aug 7-13th for all three scenarios:
Nashville to Anchorage roundtrip $672
Nashville to Anchorage and return from Vancouver $1000
Nashville to Vancouver and return from Anchorage $649.
See, the roundtrip in this case is actually slightly higher than the two one ways. All you can do is price it out and see what works for you.