Sabotaging their project has collateral damage to the company and to everyone else who was involved with the project or depended on it—unless it was a one-person project that didn't really matter. Collateral damage to innocents is bad form.
If A's revenge took the form of damaging B's car (not while anyone was in it) and costing thousands of dollars in repairs... Well, it would depend on just what B had done to A, and what alternatives A had. But if B had previously inflicted the same magnitude of economic damage on A, and perhaps gloated that there was no way for A to prove it, then... I think the main thing I'd feel for A is respect.
If A's revenge took the form of damaging B's car (not while anyone was in it) and costing thousands of dollars in repairs... Well, it would depend on just what B had done to A, and what alternatives A had. But if B had previously inflicted the same magnitude of economic damage on A, and perhaps gloated that there was no way for A to prove it, then... I think the main thing I'd feel for A is respect.