Your best bet would be to:
- Remove subfloor in attic
- Air seal at ceiling level around all wall intersections, top plates, plumbing and wire penetrations and any other gaps that are up there
- Fir up existing ceiling joists to ensure a proper depth on insulation can be installed
- Install new insulation -- blown fiberglass or cellulose
- Reinstall subfloor
I would not be concerned about the lack of a vapor barrier -- new research has shown that they actually do more harm than good.
For more information:
Read Lily Livingston's Q&A "Doesn't sprayed cellulose insulation tend to settle, leaving areas in the wall where there is no insulation?"