There’s a setting that specifies what order your lessons will come in. The default is by level and then type. So it will always do earlier levels first, and then within a level it will do radicals, kanji, and then vocab. So what you are seeing is the level 1 vocab, and once you finish those you’ll see the level 2 radicals. You can change this setting if you want to add some randomization, but you cannot select your lessons with vanilla WaniKani.
However, there is a script I built specifically for this use case. I used it to learn radicals, kanji, and vocab together all at the same time (I usually learned all the radicals right away since there aren’t that many at higher levels, 3 kanji, and 9 vocab per day). This script is much more flexible that Reorder Ultimate for the use case you’re describing.