What year Vitara are we discussing and how are you determining in that the front tires don't spin? Do you have auto or manual free wheel hubs?
The Vitaras have a very simple and for the most part reliable four wheel drive system.
Jack one front wheel (right front preferably) off of the ground, block the vehicle so it can't roll and lock the front hubs.
Select 2H on the transfer case and reach under the car and try to rotate the drive shaft (transfer case to front differential) - it should be possible to turn it - try to rotate the front tire that is off the ground - it should be possible to rotate it.
Select 4H on the transfer case and repeat the above tests - neither the drive shaft nor the wheel should turn - if the drive shaft turns, there is a transfer case problem, if the wheel turns the problem is either in the front differential or one of the free wheel hubs - you can determine which by holding the CV axle shaft with one hand and attempting to rotate the wheel with the other.