Will different diameter exhausts make any differance in manifold vacuum???

It can, but it mainly would depend on valve timing and the effect of backpressure present in system and it's effect on cylinder filling...
Intake system design, reasonable valve overlaps and ignition timing can have a more controlled effect on exhaust scavaging and result in higher vacuum levels than exhaust system size in many cases.
A slightly 'small' system can help low-end torque to some extent when combined with larger vale overlaps by reducing cylinder scavaging and increasing combusion temps. But will result in lower cylinder filling efficencies at higher rpm levels and loss of output.

If you are looking for the optimum size for a given engine, go by - 60Hp per Square-inch of cross-section.
This will give you a decent lower end torque and not have much negative effect on output at traditional upper rpm limits for street use.
You can 'sort-of' figure that a system that's 2.5" = @275Hp/2.75" = @325Hp/3" = >400Hp if you're looking for a ballpark sizing structure.

Choice of muffler type, system bends, crossovers and other confiquation issues can effect the system also.

Hope this helps some.