18 to 21 inches of vacuum is factory on all engines.
IT starts @ the intake manifold & ends on the front axle.
If this happened all at once, check the vacuum lines. A cracked or broken line will cause high or rough idle.
The 02 Chevy S10 with a 4 cylinder or a 6 cylinder does not have a timing belt.
You failed to mention what year the S10 is but, most of them use a fusible link to protect the alternator circuit.
Considering the Silverado has a 6 lug rim and the S10 has a 5 lug rim, it would be an expensive project to convert the 5 lug S10 into the 6 lug pattern of the Silverado.
Test it with a vacuum gauge.
GOOGLE!
Look under the battery tray.
check for a vacuum leak.
It doesn't have or need a line connected to it. The spider is inside the vacuum chamber.
spagettie
you may have a vacuum leak somewhere.
Some of the HVAC controls are vacuum operated so, yes a vacuum leak could effect the cooling.
http://ui.abimg.net/images/answers/598789/2282485/1988.S10.Blazer.Vac.jpg?1328637075a
on the top of the transmission its on the transfer case with 3 vacuum lines coming out of it.
It is a vacuum reservoir to keep the HVAC controls in operation when engine is not producing good vacuum such as under acceleration or climbing long grades. Newer S10's still have one but it is hidden inside the drivers front fender.
A plugged catalytic converter could do that.