I don't know how useful this information will be to anyone, but I'm hoping at the very least it can give some insight into the bug... I went into vault 3 two times, and in the second time I got the DLC load error. I was collecting evidence for the quest "Heartache by the Number" and told Cass to wait because I didn't want her attacking people. For some reason, I decided to make a detour to Vault 3 by myself along the way. I found the NCR ranger and killed Motor Runner, but there was so much loot that I couldn't carry so I got Cass and came back to get it. After leaving the vault and heading back to Novac my game froze, and upon trying to load it again I got the DLC error. I have a lot of saves so I loaded up a recent save (tried several different ones) and continued on like normal, but every time I saved the game and tried to load it I would get the error. Eventually, it would say "Some of your downloadable content is corrupt and could not be loaded" as soon as the menu came up and all of my files except for the earliest one got corrupted.
This was over a week ago, and I had mostly given up and decided to wait for a patch, but decided to try again anyway. Two days ago I cleared my cache, installed the game to my hard drive, turned off autosave, downloaded the latest patch again, and when it started I didn't get an error on the menu screen. All the saves I had after going back into the Vault with Cass were still corrupted (1 worked but every save after playing it wouldn't load), but the ones from before I went back were somehow working again. I loaded one up and continued playing, saved and loaded with no error. After getting 2 hours of progress I save and load and its fine, but after turning on my xbox again later, I got the "some of your downloadable content is..." message and the new files were getting the DLC error again. I cleared my cache again, downloaded the patch again and this time they loaded normally. I have been playing for a while now without the DLC error coming back. I'm not sure how much vault 3 has to do with this bug because the point where I dug myself out was after I had gone in once already, and I still have motor runners helmet stuck in my inventory permanently as of now. Maybe I got really lucky, just doing random things and now my game works, but having a lot of saves to fall back on definitely helped for me.

how do you clear the cache and redownload the content?

On the dashboard go to system settings > memory > you will find option to delete the cache > this will delete all updates so you will need to update all your games when you load them up again.

yeah, you can hit y on memory to bring up the option to clear cache