In general, we usually advise applicants to wait a year rather than rushing and putting together a less-than-perfect application. I think the same advice applies to you.
But, I wonder what exactly a year's worth of volunteering will do to help your application. It would help in that it would show your community involvement, but does it tie to your longer-term story about why you want an MBA? Don't get me wrong... I don't mean to make it sound like you should only volunteer if it would help you get into school. I think it's great that you want to do this! But you will need to be careful to aviod looking like someone who doesn't have a clear goal in mind.
You mentioned that the "Why an MBA?' question is a weakness in your app. I would therefore focus on trying to bolster this part of your application over the next year. I'm not sure if volunteering would help. Maybe it would, but I don't know. Maybe a better option would be for you to stay in your job and to try and take on new projects and bigger leadership roles.
BTW, your GMAT is fine. The verbal score is kind of low, like you said, but strong essays and good a pefrormance in your interviews should help to answer questions about your English skills.
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