For me, answering questions and voting on questions are independent. I tend to view giving an up vote as something like making a statement like “Hey, great/useful question.” (likewise for answers).
Just because I am able to decipher a vague, ill-formed, and/or confusing question and supply an answer does not mean that I think it is a “great question”. Likewise, I almost never consider questions that are “RTFM-answerable” worthy of an up vote (even if I post an answer that summaries the relevant bits of documentation).
On the other hand, if I find a question or answer concerning a topic that I find important or interesting (roughly “useful”), I might give it an up vote even if it lacks (a bit of) clarity.