Re: Smallest boat to make it to marthas Vineyard
First, I commend the OP on doing so research before just jumping in and going.
While the boat does play an important part in getting you there. Certain boats are far more seaworthy for that crossing then others. But, don't forget about the other parts of the equation. The weather and YOU!!!!!!
Do you have enough experience to do this trip? Can you read maps? Have you handled your boat in rough weather? In general, how good are your seamanship skills? Do you have spare parts and the knowledge to troubleshoot a problem in rough weather in the middle of the crossing? The captain of the boat plays a big part into this equation.
Now, I am not saying that you can't handle it, only you know that. I am just trying to throw out a reminder that even the best boat with the worst captain might not make it. Likewise, a smaller boat with a well acomplished captain could very well make it safely. (Such as JB

)
You must know your limitations. There is always risk involved, learn how to decrease the risk to a comftable level for you. This will be different for everyone. This may include using the right boat, practicing your seamanship skills in that boat, having all needed safety equipment plus some, filing a float plan, doing the trip with other boats, learning to read the weather, building extra days into your schedule for weater, etc.
Maybe you have already thought about this, I don't know. Whatever you decide to do, please do it safely. Don't make the men and women in the Coast Guard risk thier lives to save yours because you were ill prepared for such a trip.
Good luck!