When using the pneumatic stapler you don't have to worry about length; it won't matter much if you hit the joists. 1" is a good length, even 1 1/4" is fine. You can use 3/4" if you want...all you're doing is laminating the underlayment to the subfloor. Also, don't worry about hitting a nail or screw below; most of the time the staple leg is deflected and bites adjacent to the obstruction. In those cases where you do hit a nail directly, the staple will just bunch on the surface and is easily removed. You should use the fastener schedule for the panel you buy. Most panels will have the schedule printed across the panel and at the edges. Just place a staple at every mark and you're good. Stagger the seams of the underlayment and the subfloor. I hope you've purchased an approved panel or intend to. Luan is not suitable, so if you've bought that stuff, take it back and get a birch or similar white wood panel made as underlayment. After your underlayment is down, only use a cementious patch/filler where there is a significant gap between panels. Simple light sanding to smooth panel edges to make them flush is all that is required. However, If the seam edges are pulled down enough to become a dip or trough in the surface, then use a filler to bring that up flush. Your success with this will be in your efforts to create a smooth and flat surface...that is your goal.

You MUST HAVE a vapour barrier installed over the concrete. All floating floors have this requirement. Vinyl is NOT to be used as protection from moisture migration UP! You can purchase 6mil vapour barrier + moisture resistant tape (aka Red Duct Tape) for the seams. Should cost about $0.12/sf for the space. And NO....you CANNOT use another "underpad" under that vinyl. You can ONLY USE polypropylene plastic sheeting. Vinyl does NOT like padding underneath. You must work with something thin WITHOUT padding or cushion. Go cheap and cheerful. The 6mil vapour barrier (as above) is considered "Best Practices" in the flooring industry. No need to reinvent the wheel.

No you cannot use a soft underpad under vinyl - especially one that already has an integrated cork backing. Vinyl does NOT like underpad....ever. There are some of the thicker products like CoreTec that allow for it (2mm cork is allowed under CoreTec 1....but none of the others because of their cork backing). You have few choices here. I'm going to guess you have sheet vinyl or vinyl tiles. Linoleum (the real stuff) is quite rare and very expensive so it isn't an 'every day' product. Sheet vinyl rarely had asbestos in it...but sometimes the cut back adhesive had asbestos in it. The vinyl tiles (especially 9"x9" tiles) were the one's to have asbestos issues. That being said, you won't have any problems if the vinyl/lino is dated from the 90's. Asbestos was 100% banned by the late 80's. Anything built after that should be lead and asbestos free. That means you can take a quick look at the age of your house to determine the LIKELIHOOD of asbestos (including popcorn ceilings, drywall, blown insulation, etc). Some pictures of what you have would be REALLY helpful. Some of our flooring professionals on the forum can 'spot' asbestos containing products a mile away. Of course the easiest course of action = small T-molding for $15/linear foot and you are DONE like dinner! Sometimes cheap and cheerful wins the race.

To follow up, I found out that Home Depot carries MSI products that are the exact same product I am looking at under a different name. They call the MSI Prescott series of flooring their "Heritage" series on their website but it's special order for in store, but MSI's Cyrus series is available in store (under a different name as well.) So I went in to HD and spoke with the flooring manager. He confirmed they are the same product, just branded for their stores. But, he also told me it's not recommended to put any underlayment or vapor barrier under any of the the MSI flooring. When I questioned him on the vapor barrier, he tried to sell me on their LifeProof flooring, saying that was the only vinyl plank flooring with an attached pad you could use any kind of underlayment on (sorry, I'm not interested in a more expensive floor with 1/3 the thickness of the wear layer.) I then emailed the company I originally found the flooring at online (their prices were better than HD anyway and they are offering free shipping to my home.) I asked them about the underlayment and they said "The msi material you cannot add any additional underlayment under this product, it will break the locking edge over time." So then I asked for a quote for the flooring with just the Visqueen 6Mil Vapor Barrier and they replied "Here is the order you requested minus the Visqueen. This material has a pre attached pad which has the pad and vapor barrier." And they only quoted me on the flooring. I can't find anywhere on MSI's website that says a you should or shouldn't use a vapor barrier (or underlayment for that matter) except the line in the installation instructions saying "This product is waterproof but is not a moisture barrier. The product can withstand topical water and water penetration for up to 16 hours without being damaged. However, water leaking over or around the outer edges of the flooring can damage a wood subfloor and breed mold/mildew growth on subfloor and walls. This is not considered a defect in the flooring." So I emailed MSI directly asking if installing underlayment or a vapor barrier would hurt the product or void the warranty, but they didn't answer the questions and just sent me to a link to their installation instructions (which I had already read to no avail.) Maybe I am going on a wild goose chase looking for the answer I was expecting saying I should install a moister barrier over my concrete floors. Install would be easier and cheaper without this step, but it seems odd to me not to put any vapor down. Regardless of what goes under it, I just ordered the flooring and was told it should be here in the next couple weeks. Am I crazy and trying to overcomplicate this install by wanting to add an 1mm extra pad or even a vapor barrier?