Yeah, the previous posts are right about the math.
More technically: the optimal thing to do is reroll anything that comes up less than average. As keterys said, for d8s this pops the average result from 4.5 up to 5.5. However, as also noted, because of your rerolling, you're more likely to get results of 5 and higher than 4 and lower -- in fact the ratio is 3:1. Thus the result is both more consistent (the deviation from the mean is smaller) and skewed to higher results, as compared to a d10.
More numbers as an example: with supremely vicious, and rerolling d8s on 4 or lower, you'll get a 6 or higher about 56.25% of the time (compare to 50% with a d10). Of course, rolling 9 or higher is impossible (but happens with 20% of d10 rolls).
The real win, as keterys also pointed out, is that "supremely vicious" also lets you reroll any other crit dice too, such as what you get from the "high crit" property, and these can be d12s already. If you have Devastating Critical and have a +3 vicious d12 weapon with high crit, then your extra crit damage is 5d12+1d10 (average 38). With the same feat and a +3 supremely vicious d12 weapon with high crit, then your extra crit damage is 3d8+2d12+1d10 with the ability to reroll, which pops the average up to 41.75. So you get a 10% increase in average damage, and it'll be more consistent and skewed high on top of that, so that's a pretty good advantage.
Just to be clear, the advantage doesn't really depend on paragon levels, it's just that Devastating Critical makes it more obvious.
Short summary: without high crit plain vicious is better. With high crit on a d12 weapon, supremely vicious is better. It really takes off if you start tacking on extra crit dice in other ways, too.