Zero both your scope and BUIS at 50. That gives identical holds. Otherwise you will have to dial your elevation turret to 200 to have a 50 yard zero, which takes it off of the lock.
If you keep a 100 yard zero you will screw up your close range hold overs when using 1x up close. This also frees up a few extra clicks on your elevation turret for the BDC to reach 600 on the first rotation, no matter your ammo choice.
You have zero reason to choose a zero distance other than 50 yards for both that optic and your BUIS.
There was no need for the 20 MOA mount. It just puts your optical center, which has the best image quality, only available at 500+ yards instead of point blank to 100 yards. But you probably won't notice anything negative with that scope.
Do not kid yourself and think you will shoot 5.56 past 600 yards. It sucks for that. Especially in any wind at all. A zero MOA base would have dialed you to dial 800 with that scope in 5.56 even with a 16" barrel... and past 800 the 5.56 bullet goes transsonic and gets unstable anyway. 20 MOA bases are for shooting beyond 800 yards and usually for beyond 1000 because a 10 MOA base gets most guns to 1000 that will really be effective at 1000.
A good rule of thumb for base selection:
0 MOA base = under 800
10 MOA base = under 1000
20 MOA base = over 1000
And finally... a 1-6 VX6 is a great optic and will be great... to about 300 yards. Past that, you will be wanting more magnification, a bigger objective, and adjustable parallax. Well, if you care about group size or shooting at 2 MOA targets. If you only shoot torso sized billboards of steel, it will work to your intended distance of 600 just fine.