Shapes can be drawn over DEM or Orthomosaic as well as over model using instruments available on the Ortho/Model view toolbar. The Snapping tool in Metashape will help you to do vectorization. This solution describes how the snapping works in Metashape Professional version.
The following snapping modes are available: Axis, Vertex, and Edge. Modes selection is available from the dropdown list of the shape drawing selection button on the toolbar of the corresponding view:
To use snapping mode, enable one of the available options: Axis Snap, Vertex Snap, Edge Snap on the toolbar and hold the Shift key when you start drawing shapes.
Axis snap - snapping is done along the coordinate system axes:
Vertex snap - allows you to snap the vertices of the shape that is currently being drawn to the vertices of the existing shapes:
Edge snap - allows to snap the vertices of the shape that is currently being drawn to the sides of the vectorized objects (already drawn shapes):