Should we use regular plywood or Lauan over fir strip sub-floor before laying vinyl?
Well technically it is plywood, it's just not very good plywood. Having said that I've seen flooring guys use a product that looked very much like luan that was no more than an 1/8 of an inch thick to get the level they needed. This was in a hospital remodel so it was definitely up to code and it went under a commercial grade thermo-sealed floor so it should be good for just about any application unless your subfloor is really atrocious. Ask around at a dedicated flooring supply store and they should be able to point you in the right direction.