I do the same thing but sometimes need to tweak the resulting roof faces a little after import and find it utterly irritating to do so, so I find myself back in Sketchup doing the tweaking. For me, there is no comparison editing the roof model in Sketchup compared to dealing with VW's roof tools. Time is money!
To share my discoveries with the Sketchup to VW import tool, I have documented a few important things to do to the Sketchup file prior to import, if all you want is the roof.
Setup a Sketchup drawing template to have a set of layers with names that match the target VW design layers.
1. Either make sure the roof surfaces and edges are on a separate layer, preferably on a layer that matches the Vectorworks target design layer, and have all the faces assigned to a specific Sketchup material.
2. Un-group all the roof faces, VW sees grouped polylines rather than roof faces if the Sketchup roof surfaces are grouped.
3. Move all the faces to the Sketchup Z drawing origin (not an origin set with the axis tool) if you only want to use the roof surfaces and have them come into VW at a zero Z axis height and relative to the roof design layer Z dimension. Vectorworks will import the file, convert the surfaces to roof face objects and place their Z axis elevation at the Sketchup drawing default of 0 Z.
4. Make sure your Sketchup roof design takes into account the desired soffit depth when using Vectorworks' exterior wall components beyond the structural wall.
Don't get me wrong here, I love VW and all it's capability, I just think, for me anyway, designing roofs in Sketchup vs. VW is an exercise in real creative, and time efficient, design work rather than just killing time fighting the software. Thank goodness for the Sketchup import filter!