Per Einarf (ModernGL dev) on Discord, not being able to add buffers to a VertexArray after it's created is a limitation of ModernGL at the moment,

You must create a new VAO for each of your VBOs, or re-create the same VAO with the new VBOs each time, which isn't even worth the complexity, because VertexArrays are small metadata objects, as Einarf says.

I was seeking an answer to this merely because I wanted to be as correct as possible with my code. I ended up just having a VAO for each VBO.