If you bought the model that came with the picatinny mount, VMD-3103, all you have to do is buy a riser, which is low cost and will get you were you need to be. Vortex is selling a .6" high adpater (pretty costly) and saying you will a 1/3 lower co-witness. So you could just buy the appropriate length picatinny spacer and get the same for a lot less (fleabay). Stay away from the cheap air-soft Chinese stuff and get something of better quality.
If you bought the sight with no mounts I'd go with their adapter Mount Riser-QR, which is a direct attachment to the top rail on the 8500, and also gives you 1/3 co-witness.
I'm guessing you want to use the irons and red dot together, however, if you don't carea about a co-witness. Put some cardboard or other solid spacers under the sight and get it to a comfortable height for your shooting style and look to see if it will work for prone and standing, then buy a spacer that gets you close to that dimension.
I'm not mounting a red dot, but a scope on my AR and as an example to get a 2" above rail to scope center-line using a GG&G GS-1 riser .5" and a Geissele scope mount 1.54"
