Does this have to be attached to a tent or can it just stand on it's own?
It must be used with the tent. It does not stand on its own.
Believe it or not, I bought the SL3 vestibule as an addition for my Eureka Apex 2 person, and have just bought the Eureka Forte SQ. I can use it w/ either, I measured the front width of the tent to make sure the vestibule would match. I bought the Sierra Grip clips and have successfully found way to attach it to the frame before I put the fly on. I have it attached across the front of the tent in 5 places, Center to pole overlap, then midway each side.... [attaching to Rear poles,] and bottom corners down to front. The tent fly corner wraps over the vestibule and secures midway btw the vestibule center stake and outer corner. Worked perfectly. The only small drawback is that the SL3 has a zipper designed to attach to the Big Agnes tent, so the shape is slightly different. After attaching the grips clips near the corners, I just rolled it up and tucked it under. I got it at an extreme discount, so that minor detail is not an obstacle. I bought a 6 x 8 tarp that fits perfectly under the tent and continues out under the vestibule. Great combo!!! With extended inclement weather, 2 people can be tucked under with plenty of room!