Brett, welcome to Ice in Space
It will really depend on the specs of the flattener/reducer. If the reducer presents an M42 thread to the camera, it will almost certainly result in some vignetting. Whether or not it’s manageable is difficult to answer.
Most flattener/reducers designed to cope with a full frame DSLR will be on the larger side and present an M48 thread. T-rings for full frame cameras screw on to this.
The other complication is whether or not optical issues with the scope or flattener muck up the stars towards the corners. If so, then the vignetting is likely moot and you can just crop away the ugly bits.