Now this is relying HEAVILY on speculation, but I think it is pretty acurate, and tbh the most accurate you can get with this sort of stuff.
Now the Superman vs. Goku Death battle made by Screwattack was extremely technical and accurate, and it should give us a clear idea of how to compare Vegetto to Superman. Goku in SSJ4 form wasn't able to surpass any of Supermans abilities. it goes like this...
Strength: 160k Tons (w/o ki)
Speed: 2.5 Billion km/hr
Durability: 34.7 sextillion MT
Strength: 6.6 quintillion tons
Speed: 9.4 Billion km/hr
Durability: 10 octillion MT
so we can see here, Superman far outclasses SSJ4 goku. but what are vegetto's stats? this is what requires speculation. we will also be assuming Vegetto can go SSJ4.
Supreme Kai stated that Majin Buu or "Fat buu" is a thousand times more powerful then freiza. and freiza had a powerlevel of 120,000,000 so that gives Majin Buu a power level of 120,000,000,000. Majin buu then later expels the evil within him creating Evil Buu, who was a little bit more powerful then Majin Buu, and judging by the battle, Evil Buu can be no more powerful then 130,000,000,000. Evil Buu then absorbs Majin Buu, adding his power to his own, making Super Buu with a total of 250,000,000,000 power level. later on, Super Buu absorbs Piccolo and SSJ3 Gotenks. SSJ3 Gotenks was stated to have the same power as Super Buu, and judging by their battle it would appear to be true, although some believe that Gotenks is stronger and Super Buu only leveled the playing field with his regeneration abilities. but we will just assume that they had the same power level, and with piccolo as well who probably barely added any power at all, we would have a power level slightly higher then 500,000,000,000. this form of Buu was able to fight on par with Ultimate Gohan, so we will assume that Ultimate Gohan (or mystic gohan as some call him) had a power level of 500,000,000,000 as well. the fusion between Goten and trunks (who create Gotenks, for those that don't know DBZ) wheres off inside Super Buu, thus returning his power level to 250,000,000,000. but since Buu does have piccolo and the two saiyans in him, we can judge that his power level is around 260,000,000,000, maybe more. Buu then absorbs ultimate Gohan, and thus becomes the most powerful villain in DBZ, with a power level of roughly 760,000,000,000.
Now Vegetto (fusion of Goku and Vegeta), while in his base form, was able to fight roughly on par with Buu, but many say that Vegetto was more powerful then Buu, some say vegetto far surpasses his power even. but I'm just going to assume he's on par, sounds fair right? so that puts Vegetto at 760,000,000,000 power level in base form. now how do we compare to Goku? well SSJ3 Goku was able to pummel Fat Buu, and was stated by piccolo that SSJ3 Goku could kill Buu if he really tried. Goku was also however able to hold out to Super Buu with Gotenks absorbed for a short period. Goku also had almost the exact same power as SSj2 Majin Vegeta, who was able to put up a decent fight against Fat buu, but Fat buu later gained the upper hand. so I'm putting SSj2 Majin Vegeta at 100,000,000,000 and since SSj3 is 4x stronger then Super Saiyan 2, that gives SSj3 Goku a power level of 400,000,000,000 which I think is about right. now divide SSJ4 Goku's stats by 10, and you got SSJ3 Goku stats. Base Vegetto is 1.9x stronger then SSj3 Goku, so multiply all of Goku's SSJ3 stats by that, and then multiply by 4000 which is the multiplyer from base to SSJ4, and you got!...
Strength: 121.6 Million Tons
Speed: 1.9 Quadrillion Km/hr
Durability: 26.37 Octillion MT
Now that we got Vegetto's stats, we can discuss who would win this "Death Battle".
please watch the Goku vs. Superman to give a clear idea on how this battle works and some of the abilities of superman wich I won't go over. http://www.youtube.com/watch?v=oyl97TG8jbA