I definitely say layers are the way to go!! We went to Alaska last year, but we went in May. We had all different kinds of weather. It ranged from cold and rainy to sunny and HOT!! Our day in Juneau we were bundled up for a day of whale watching and it turned out that we were all so hot in our jackets and sweaters. The locals were even telling us how unusual it was to be that warm at that time of year. I mainly wore jeans with a long sleeve shirt and then a pullover or sweater type jacket on top. Then if it was really cold I would put my regular jacket on top of that.
We went from Seward to Vancouver also. Will you be taking the train from Anchorage to Seward? I LOVED Alaska!!