Do you have to use a moisture barrier when gluing finished wood flooring on concrete, if so what is the best product to use?

You need to test the concrete to find out. Many will put them down on all floors no matter. as far as systems their some concrete guru's on here that have more experience with those products. im in wooden subfloor country so we only glue down as a last resort.

I'm just finishing up my first glue down on concrete and I can say this- HAT'S OFF TO THOSE OF YOU THAT DO THIS FOR A LIVING This was 5 times harder than staple/nail down IMO....

Anyway, do you need a barrier? I think most manufacture's are going to say yes. You still need to test for moisture content and fix any issues that may arise from drainage issues, etc, but likely you won't have any....

I did use a vapor barrier for piece of mind. I suppose if you're putting down a small amount of flooring in a low traffic area you could roll the dice but if you're doing 300 sq. ft. or more then it's probably not a good idea not to use one.

You can use a two step process, the vapor barrier applied using a trowel then afterwards glue down the flooring over that with the adhesive, or you can use a "single step", or "One step" adhesive that has both the vapor barrier and adhesive combined and you spread it only once to install the flooring.....

It depends on the type of glue that had been used. If the glue is dry, you can use a buffing machine with a sanding disc. If the glue is rubbery, you will have to scrape it off. Do not use any kind of liquid stripper; it will turn into a soupy mess, which will be a handful to take care of.