Where can I find a VTAC sling that costs $25??
carries them for $29.99 regular price, and if you watch for a sale or coupon code you can do even better still. I purchased both of my VTAC slings at the local uniform and equipment store for $25 plus state sales tax, so look around locally and you might find a similar price.
I've shot Appleseed and appreciate that the USGI sling can be used to provide a stable shooting platform. Where the VTAC excels however is as an actual sling intended to carry a rifle. The quick adjustment of the VTAC is far superior to the adjustability of the USGI sling, and this feature allows for quicker adaptation of the rifle to a variety of carry positions and also quick adjustment to shooting positions when the sling is used for support. This is a feature I absolutely want when I'm carrying my rifle in the field so I can secure the rifle, quickly deploy the rifle, and quickly adjust my sling to provide shooting support when needed.
The other feature I find superior on the VTAC is its method of attachment. While the aforementioned metal clip of the USGI sling is nice for quickly detaching the sling when using it as a shooting aid, I don't like that feature at all for actual carry of the rifle. The fact that it quickly detaches also means that it is prone to accidentally detach under heavy use, and for $1000-1500 of rifle and optics I much prefer something more secure even if I'm doing little more than hiking through the woods. I also don't like that the attachment is a metal clip, because metal-on-metal attachments rattle and make noise I don't want to make while I'm out stalking Wile E. Coyote or Bambi.
So again, if a USGI sling meets your needs the have at it. Personally, I find the USGI sling to be little more than a glorified carry strap that is woefully inadequate for actually carrying a rifle, and so I choose not to use them.