你既然看到问题要提出来。

建议加强跟你的经理的沟通。告诉他一you are concerned that 因为客户要求不明确，可能会造成rewrok，最后超budget。 suggest put more effort on clarification. However, sometimes, even the client doesn't know what they want. It will be problem for a fixed budget project. You may not be able to resolve, it, but doesn't mean you shouldn't be putting the effort. It may be your manager's job to do that, but your feedback to him is important for him to do well on his job.
你说的：“这个工作是合同，但是按时间给工资很及时。如果以后真遇到问题完不成我好像没什么损失” 可能对可能不对，将来有没有损失谁也不知道。但是既然他们雇你为这个项目做事，你是不是应该负责任的把事情做好，instead of只考虑自己拿钱有没有损失？it is your call...