and the reply is silly.. and many comments point out its flaws.
If an answer is wrong or unhelpful, even if it attempts to tackle the question, it should be downvoted or commented. Assume good-faith, but try to help the answerer by adding comments about what they did incorrectly.
Then, there's another reply, this one is useful, it's not an answer and doesn't pretend to be an answer, but it's useful. Should it be given a vote?
If it is "Useful", then you should absolutely give it an upvote. Unless they are specifically ignoring facets of the question. (I.e. Q: I'm using Windows, with a problem XYZ. A: Switch to Linux and use ABC) If the answer could be useful to the poster, then you should upvote it.
This, however, is a fine line that I've clashed with the community about before. If the answer does not answer the question, then it should have to meet a much higher standard to be upvoted. Mostly because it is incredibly frustrating for a poster to watch as unhelpful "answer" gets upvoted, and no useful answers appear.
But, if you like the answer, then you upvote it! That's how you show your approval until you get more privileges to show it in other ways.