The trend at the time was for tail-mounted engines on trijets. Many of the development drawings that preceded the final VC10 design show trijets with a Trident style engine installation (a lot of these were Vanguard-based and named 'VanJet'). So I don't think anyone would have come up with a trijet with two wing-mounted engines. It would have meant a taller undercarriage as well with all the weight penalties that this incurs. And also, the engine to do that for a VC10-sized airliner was not around (as far as I know). Rolls-Royce was planning a RR Medway that might have been developed to higher thrust levels but the tested variants were in the 16,000 to 17,300 lbf thrust range, while the Conway reached 22,500 lbf on the Super VC10.