The Vortex is made by Smith Enterprise:
http://www.smithenterprise.com/
http://www.smithenterprise.com/products06.html
It looks like you have the light barrel on your XCR, so you want the G6A2 model (part #1001V) for 5.56. I think I remember ordering mine from Rainier Arms:
http://www.rainierarms.com/?page=shop/manufacturer&manufacturer=22
There are two theories as to how it improves accuracy. One, its fibrating forks stablize gas expansion when the bullet leaves the barrel making the explosion more consistant. The other theory is that since you don't have to torque it down very tight, it allows for more consistant vibrations in the last inch of barrel. Since the forks are helical against the thread attachment direction, the gases from shooting cause the the flashhider to put twisting force against the threads to keep it tighened as you shoot. All I know is, it definately works. I've put them on three different rifles and did my own testing and in each case accuracy improved.
Most good M4s shoot about 2 MOA, and I would say my XCR does that. I'm not a precision shooter, but my groupings are tighter at the qualification range with the XCR.
The trigger is a two stage trigger which means you have the initial pull and it stops, then a second pull which finishes the stroke. It's a good combat trigger design because when you are under stress, the second stroke is activated from a position of tension. So, the activation is more predictable and less jerked for long shots. With a single stage trigger your finger goes from relaxed to activation, which can result in more jerk under stress. A two stage trigger kinda compensates for the "oh s**t" factor so that your shots to suffer as much. RA does make a single stage trigger if you want one though.
My sling is a pretty high end one. It's a modular setup from Blue Force Gear:
http://www.blueforcegear.com
I honestly don't remember which model mine is. You can get a H&K clip to attach to the rear ring on the receiver, but I've found that it sometimes gets in the way of the charging handle. So, I just attached one end to the MI rail sling mount, and the other end I fixed to the stock by slipping it between the butt pad and the stock. Works great. Any modular tactical sling for the M4 should work for you, but you'll want to order a rail mount slight attachement. I got mine from Midwest Industries:
http://www.midwestindustriesinc.com/index.cfm?fuseaction=category.display&category_id=80
You can also get rail covers from them. Mine are ERGOs that came from RA when I bought the XCR.
I can't give you a whole lot of advice on scopes. I don't use them that often except for hunting rigs on bolt guns. I tend to tailor my assault type rifles for quick target aquisition at close and medium ranges. The ACOG is a damn fine scope from what I've read and it might be worth it. If it were my choice, I would get a Aimpoint red dot sight for better shooting at ranges under 100m.
As for lube, a good general purpose lube is Militec-1 for rubbing surfaces of firearms. However, you want grease which is more long term for rotating bolts and the heat buildup you get in an assault rifle receiver. I use TW-25B grease for areas like that.
http://www.milcomm.com/weapons_lubricants.html
I'm sure Walmart has some similar products, but I haven't looked lately. You want gun oil for light areas of friction, and gun grease for internal parts that go through a lot of wear. You want to be careful though, as oil and especially grease attract dust. In dusty environments I suppliment oil for a dry-on lube like Tuf-Glide which does not attract dust.......however, it doesn't take the place of grease. Anywhere where you notice parts rubbing, you want oil or grease on there. Since the bolt rotates inside the bolt assembly and rubs all the way around, you want grease on those areas.
