It's elementary. Have in A.Designer the photo as an object - pasted or imported, both are ok. Lock the photo in the layers panel to avoid accidental scaling, edits or moving.
Build your drawing aside. No tricks are needed. You can as well draw the vector on the photo in A.D if that helps. The photo stays untouched as long as you do not want to edit it.
You can group the vector parts to keep them together. Layers panel gives access to every item.
About sketching in Affinity Photo: That step is not needed, you can draw in Affinity Designer. In pixel paint mode you can paint into a pixel layer if that's wanted.
Consider to check Inkscape version 1. It has quite advanced automatic bitmap image tracing. Vectors can be copied and pasted directly from Inkscape to A.Designer.