I went down from Vincent Gap to The E Fork of the SG River about a year ago (fall '07). Eventually, the trail does peter out, and you just walk down the rock strewn creek bed. I didn't find the rock inordinately difficult, nor did I find the trail to be badly overgrown. Your mileage may vary.

There were a couple of dead falls (downed trees) that had to be negotiated. They weren't easy, but they weren't the end of the world either. An older gentleman (60's) my friend and I met was able to negotiate them with a full overnight backpack.
The trail coming down from Cabin Flat was signed. My friend and I didn't have time to explore up Prairie Fork; we just turned around and went back to Vincent Gap, but we did go up to the trail sign and could see the route up to Cabin Flat.
If you just go back up to Vincent Gap, you need to really monitor your time and level of fatigue. Climbing back to Vincent Gap isn't as steep as say the N face of Baldy, but it definitely is a climb. The downhill that you came in on can be deceptively easy.
The other thing to make note of is
where the trail finally peters out and you just follow the watercourse. You want to be able to exit the creek bed at the proper point to get back on the trail. By the way, in the lower section where one generally just follows the creekbed, bits of trail will periodically reappear on the east bank. Some stretches are worth following inasmuch as a trail is quite a bit easier to travel on than a rocky creekbed.
By the way, the old, maintained trail camp on the flats between Vincent Gulch and Mine Gulch is long gone, but the flats are still flat and it's an excellent spot to camp. For you overacheivers out there, one can go to the Big Horn Mine, descend the steep and unstable slopes below the main entrance down to Mine Gulch and then hike out via Vincent Gulch. It can be done as a day hike, but it is in my experience a very difficult hike. The hike down to Mine Gulch from the mine is for very experienced hikers in good shape only. It's a slidey, steep route; the faint of heart need not apply. The old mine carts and such that one encounters on the way down are very interesting.
HJ