<< 2. Any different between them?( just like virtuozzo /uml/freevps/vserver)... >>
Virtuozzo is pretty much the default VPS software on the market, and is the prettiest and most feature packed. It is also not freeware, so expect providers using it to be more expensive than those that are not. There really is no best software though, i's what you like best yourself, and what your provider knows best.
<< 3. is all vps software is stable when you use it? also is the free open source is stable too? >>
This is completely based on the knowledge, competance, and experience of the provider you choose...and the hardware they utilize. Good software can be mis-run, and bad software can be tweaked to run well.
<< 4. is all vps software provide a virtual OS inside so we can install anything we need?( admin if need reinstall is to remove and redo the image). >>
Most do. Virtuozzo does not let you modify the kernel, but some of the freeware VPS software packages do.