Feature 1 - Parallel Below the Shelf
The first problem I noticed with sliding leg vices is the parallel bar sliding along the shelf colliding with anything you were planning to store on the shelf. To overcome this, I put the parallel guide underneath the bench. To achieve this I reduced the height of the wheel brackets and raised the shelf. The trade off is to remove the vice you need to undo 4 bolts.
Feature 2 - Wheels
Typically people use a 'V' groove where the sliding leg moves wood on wood. My vice weighs 20-30kg, so this produced unacceptable friction. To overcome this I used 4 x 2" wheels.