I came across it after clearing out some fiends but discovered I couldn't sell it to Vendortron, although it's got a value of 4996 caps. Is there anywhere else I can dispose of it?

Please ask Quest Related Questions in the Cheats and Hints section. General is for game play discussion, not quest or item details.

I have moved this one to CHS for you.

This is an item for a quest. It is related to the Omerta's and is part of an optional way to handle a quest from their casino.

In other words, there is no one to sell it to. Not sure what happens if you grab this, then acquire the quest, then choose the route that would require this item. However there are other ways to handle the related quest, so you have not messed anything up. However I think this thing respawns again over time, or maybe that is just the fiends there.