There are quite different:
(1) ROI Analysis - this typically means that you select your ROI, extract the data, and then run the statistics on the ROI mean/eigenvalues.
(2) SVC Analysis - this is a voxel-wise analysis within a restricted area of the brain. The maps don't change, but the corrected p-values will change because you have restricted the search region.
With an ROI analysis, you can test multiple ROIs. With SVC, you have to test all regions simultaneously or correct for the number of times you perform an SVC.
The math underlying SVC is the same as the whole brain analysis, except the search volume is reduced.
Hope this helps.