Was the cold war mostly . . . An arms/weapons race? An ideological conflict? A power struggle? A race to fill the power vacuum after the European superpowers? A technological race?

The Cold War was all of those things. If you have to pick just one answer though, you are best off picking number 3. In the end, the Cold War was indeed a power struggle. The so-called "free world," with the United States as its leader, and the socialist regimes led by the Soviet Union were engaged in a contest for influence over as much of the globe as possible.
Keep in mind that this power struggle played out in ways that meet the criteria of the other possible answers. At times, the Cold War was an arms and technological race, with the United States and the Soviet Union competing for the most nuclear weapons, most advanced weapons platforms, and the best military intelligence. It was also an ideological conflict between ideas of capitalism and democracy versus socialism and communism.
Furthermore, it did start out, in many ways, as an effort to fill the void left by the defeated Axis powers after WWII. However, you can consider all these to be just part of the greater power struggle that defined the conflict. Without the forces of communism and democracy competing for power and influence, none of these other factors would have occurred as they did.