LVP Over Concrete Slab. Also, Contractor Pricing at LL.

[Jordan L.]

Hello all,

 I'm doing an approximate 1200 sq ft DIY install of Lumber Liquidators Tranquility XD LVP 4mm flooring over a concrete slab.  The house is currently my primary residence but will likely become a rental in the near future.  The manufacturer recommends a 6mil underlayment, but I've heard this may or may not actually be necessary.  From what I've read on here, there are also mixed opinions.

 The property is in the Central Valley of California, so not a humid area.  No known issues with the concrete slab other than a bit of mortar I still need to grind off here and there due to the old ceramic tile.  I'm not sure what the purpose of the underlayment would be in this type of environment other than noise reduction or a very marginal insulation value.  If I can avoid the extra hassle of dealing with the underlayment I will.  This said, I'm not a flooring professional so it's possible there's something I'm not considering.  

 Should I install the underlayment?

  Also, has anyone had experience negotiating contractor pricing from retailers like LL?  I don't have a contractor's license but I own other rentals and have made large purchases from LL in other states in the past.    

Thank you everyone.