Models?

Hello I’ve had Half life 2 for years now and i wanted to know to fix models.
For example the elite and regular combine soldiers have a one sided bullet-proof vest. I was wondering what kind of program to use to carve it so both sides can be equal. It just makes them look fat.
This side is closed
http://images4.wikia.nocookie.net/__cb20090107202713/uncyclopedia/images/a/a1/SuperSoldier.jpg
This side is open
http://pnmedia.gamespy.com/screenshots/phl/93078848.jpg

Well there’s many 3d animation/modeling programs out there

I use Softimage Mod Tool 7.5
The sourceSDK had official plugins for it

But mostly everyone else uses either Blender

or 3DS Max
http://usa.autodesk.com/3ds-max/

Valve uses Maya
http://usa.autodesk.com/maya/

They can all do the same things but the way they work is VERY different
choose wisely