Those three adj+to phrases are indeed different in meaning! "prone to" means likely to do sth. more actively tendency; e.g. Some plants are very prone to disease. Kids are all prone to eat junk food. "vulnerable" emphasize the fact of easily being hurt and unprotected, so "~ to earthquake"means sth is exposed to the danger of earthquake.Young birds are very vulnerable to predators. "subject" means a passive tendency, under certain control or obligation. e.g. We are subject to the law of the land. Trains are subject to delay(s) after the heavy snowfalls. Thus, you cannot use them interchangeably if you want to discern the implicity meanings inside. BTW, you can use those three collocations for your examples. They are all grammatical correct yet with different meanings.