We ended running out of time. BUT that is because we chose to go and see the changing of the guard at the palace. To me, that was a total waste of time unless you are there super early. We THOUGHT we had good spots, but ended up hardly seeing anything because we were over on the side...we had no idea where the festivities took place. I think if we hadn't have wasted time doing this, we would have gotten in Skansen.
I think the Vasa exhibit can be as long or as short as you want it to be. There are so many things to read and exhibits there to see, from models of how they brought the ship up, to exhibits of what the ship was really painted like (very vibrant!). I liked reading all about it, so it took us a little longer...probably 2 hours or so? I think if you cut down the time you are at both it is doable... but I think Skansen is pretty large. Remember it also takes a bit of time to go to the different shuttle stops. Depending on what the rotation is, and having to maybe go to other stops first, it might be awhile. I think I remember it reversing the order in the afternoon.
I LOVE the open air museums. They have one in Oslo that was probably my favorite part of the whole trip. It was a open air folk museum with the big authentic Stave church! Awesome.