Glue down vinyl plank flooring installation

[Larry Ashlock]

We are rehabbing rental units and are going to be putting in the glue down vinyl plank flooring.  Does anyone have any recommendation as to whether it is better to put down an underlayment over the subfloor before and lay the flooring?  Or, is it OK to place down over top of the existing subfloor?  We took up old carpeting and scraped the staples off the subfloor so it is smooth and clean.  I have seen advice how to do it both ways but wanted to know if anyone out there had any good or bad results either way.  Would want to lay down over subfloor to reduce cost but if this would only cause future issues would rather put down an underlayment now.  Thanks in advance for any advice.