i have 76 cosworth vega what year s10 rear end will fit

The Cosworth has basically an early S10 rear end but with axles machined for a 4 on 4 bolt pattern. The main difference is the Cosworth uses a torque arm and coil springs, the S10 is a leaf spring design. Early axle shafts from an S10 2WD with 26 splines will fit if you are trying to go to 5 lugs.

Knowing how hard it is to get 13" tires, possibly you should leave the 5-lug pattern axles and also done the S 10 front spindle change (5-lug) so you could go to a 15" rim with a 5-lug. The Cosworth 4" X 4" (101.6 mm) in a similar to the Cosworth OEM wheel is a task to find. But, with the 5-lug you are tied to them and cannot show the care with the OEM special rims. So it's a pretty permanent (not reversible) change. You can also, with the S 10 hardware, go to 4-wheel disc brakes - and in particular the front disc/calipers are much superior to the OEM Vega/Cosworth. But they do okay unless you start autocrossing or whatever that uses a lot of braking.