Why is it a "vexed question", rather than a "vexing question"?

If you search vexed question in the available dictionaries, you will find that it is a British idiomatic expression which, according to Collins, means

an issue that is much debated and discussed

If you look up vexing question, however, it is interesting that Google still gives entries for vexed questions, and only one or two instances of vexing treated on its own. For example, MWebster defines it as

causing or likely to cause vexation : VEXATIOUS. Ex: a vexing problem

Therefore, I believe that a vexing question is simply a question that vexes or annoys, whereas a vexed question is rather a complicated problem that has caused a lot of discussion and argument and is difficult to solve.