There is no subframe. The car has unibody construction. Some people put in a back half subframe like they do on the early Camaros and Novas when they set them up for serious racing.
The Vega has a 97inch wheelbase where the short bed standard cab S-10 has a 108.3 inch wheel base.
I'll be honest, I just can't see how it would be practical or desirable to try to use the S-10 frame under the Vega. To even think about making it work right you would have to cut the complete floor out and build a new floorboard front to back to fit the S-10 frame.
There are several outfits that sell V8 kits and Hedders for the Vega. It was a popular swap in the 70s and 80s. The kits aren't that spendy.
You would have to have a stronger rear end narrowed and it would be a good idea to put a full cage to stiffen the body as they tend to flex when you pour the power of a V8 to it and crack the windshield along with flexing the body.
Normal chassis parts shouldn't be that hard to find through normal channels.
A google search shows a ton of info on the swap and links to parts to do it.