Increasing voltage to the CPU can usually help make an overclock stable, however as you start to reach the limits of your CPU [or your temps get too high] then your CPU voltage has less of an impact on stability.
For example, when overclocking my CPU I found that while it booted into Windows at 2650MHz absolutely fine with the RAM on a high divider [making it pretty slow], Windows never made it past the black boot screen when I brought my RAM back up to ~200MHz. I couldn't push the Vcore any further because temps were high. In the end I had to settle with 2600MHz.
So basically, yes Vcore can have a big affect on stability, but it depends entirely on what is causing the instability.
SiriusB