Sketching in virtual reality (VR) enhances perception and understanding of 3D volumes, but is currently a challenging task, as spatial input devices (e.g., tracked controllers) do not provide any scaffolding or constraints for mid-air interaction. We present VRSketchIn, a VR sketching application using a 6DoF-tracked pen and a 6DoF-tracked tablet as input devices, combining unconstrained 3D mid-air with constrained 2D surface-based sketching. To explore what possibilities arise from this combination of 2D (pen on tablet) and 3D input (6DoF pen), we present a set of design dimensions and define the design space for 2D and 3D sketching interaction metaphors in VR. We categorize prior art inside our design space and implemented a subset of metaphors for pen and tablet sketching in our prototype. To gain a deeper understanding which specific sketching operations users perform with 2D and which with 3D metaphors, we present findings of usability walkthroughs with six participants.