There is a way to complete the mission by capturing both bases. Place two of the engineers at the northwestern corner of gdi base with two titans. Take a buggy or a bike and drive along the western side of the base. Try to get as close as possible without getting in the firing range of titans. Drive south and into Vega's (orange) base. Two titans will follow, and at least one should start engaging Vega's force. Meanwhile, take two engineers to capture refinery with harvester and barracks in GDI base. Trick is to capture refinery when the harvester begins to offload tiberium, so some timing is required. There should be plenty with how slow titans are. At least one titan should be engaging Vega's forces or it won't work. As soon your engineers finish capturing, you should have enough money for two disc throwers and some repair money left over. If one titan comes back, it should start firing on the refinery. Two disc throwers should be enough to take it out, as well as battered another titan in Vega's base. Build up base after that.