Vito's are supposed to be pretty efficient, for a 2.2 anyway. But if you look at owners reviews, they are not rated very well for reliability, I guess a lot has to do with how the van is treated though.
I have had my transit connect for over 3.5 years now, not had any problems re reliability, but it isn't the most efficient van of its size. I drive pretty gently most of the time and get around 40mpg.
Not sure what sort of budget you had in mind, but the 2300 combo's are currently starting at about £8k. 850kg versions of the partner/berlingo are starting just over £4k with reasonable miles (up to 60k), the standard versions of the current shape doblo are about the same.
To be honest, if your vito is not giving you problems, I would stick with it because buying a 2nd hand van is a bit of a lottery!.