Alright so I'm new to the Vivid myself and i don't this is really my first post on here. I had the same issue with the speaker coming on automatically and couldn't for the life of me figure out what the issue was since my phone was only about a week old. This may not be the case for everyone but it worked in my case.
I had just recently downloaded Tango to chit chat with and when i was using it, had the speaker phone option turned on. Well apparently when i quit using the app I didn't turn the speaker phone option off. I read another site that said that might be the issue so I uninstalled Tango and taaadaaa! it worked. Apparently there are other apps that may have utilized the speaker phone option as well that were causing the same issue, so if you recently downloaded an app like that you may try removing it and see if that fixes the problem.
If that doesn't fix the problem then i just wasted my whole 15 minute break typing this out and you will realize why I'm still new to the Vivid. haha
Hope it works folks!
