I have relatively extensive experience in both places.
If I had to choose, I would choose Meridian.
DS, HD6 and some other natives may feel differently, but Vicksburg appears to have suffered a precipitous decline over the last decade. The public schools, aside from the local magnet elementary, are the most obvious problem. When I graduated from Vicksburg High School in the mid-1990s, it was a decently well-integrated school that offered academic and athletic opportunities on par with most other public high schools in Mississippi. It has much, much less community support than it used to, and I read in the local paper a few months ago that it hasn't produced a National Merit Finalist in more than five years.
Meridian has its problems but seems to do a better job of retaining natives and attracting young people with families, perhaps because of the local health care industry. The Riley Center usually attracts a few decent musical acts per year, and I think MCC and the local MSU branch do a good job of offering continuing-education opportunities. Also, despite the prominence of the aforesaid health-care industry, Meridian seems to present more diverse job opportunities than Vicksburg--where you can either work for the federal government or inherit your father's business.
Vicksburg still has much more historical character and better restaurants, but those things don't figure prominently into most calculations of livable-ness.