Happened to encounter this post and cannot help providing you with a simpler answer as a Chinese teacher and a linguist >_<. Hope you don't mind. I know it's already marked resolved.
1. When 对于 and 对 are used as prepositions (介词）, they are completely interchangeable. 对于 is just more formal than 对. (对这件事你有什么看法？= 对于这件事你有什么看法 What's your opinion with regard to this? 我对/对于学外语没什么兴趣。I am not interested in learning foreign language.）*In some formulaic expressions/fixed phrases like the second example, people tend to think 对 is more correct than 对于. However, that is because they are just more used to one version. If someone with a foreign face goes with the less common version, it is very likely that you will be corrected. But believe me, you are not wrong. It's overcorrection. As long as it is a preposition, they are ALWAYS interchangeable.
2. However, 对 also has the function of a verb. When 对 is used as a verb, it cannot be replaced by 对于, as the latter one is a preposition only. 对 only has two possible meanings when used as a verb：
2a. 对 as a verb meaning “direct；target; face toward”: 别对着别人打喷嚏。（pen1 ti： sneeze）
2b. 对 as a verb meaning “treat”：他对他的女朋友不好。