Maybe this is just me... but if your MIL is neglecting other kids, (and admitting it openly,) why on earth would you leave your little girl with her? You've seen major issues with her parenting 'style.' (Leaving medications and ointments out in the open and not changing a child for 8 hours, isn't a 'style.') If Vegas is that important to you, Go. But find someone else to watch your 2 year old.
Leaving your daughter in your MIL's hands will not HELP the situation. She's not going to come to her senses because your daughter is around. She won't shape up just because she has another child in her hands. She's already proven that she doesn't intend to change her ways and it's obviously fine for your husband's sister's boys. (Or not, and perhaps that's why they act how they do.)
So you can't leave your daughter in better hands because your MIL will act like a middle-school girl and try to get the rest of the family to hate you? (Screw her.) Sounds like you've got your BIL on your side of the court. Whatever you do, do not LIE about why you don't want your child with her. (Although you can probably throw a white flag and word it nicely.) If you lie, she'll probably find out and then she'll have a reason to be hurt. Don't give her a reason. Be the best parent you can be and state that your parents have offered to help, and since your daughter is more comfortable with them, (they come visit, have watched her before without incident, etc.)
If this is an issue where your husband just asked without asking you, (it happens... men. Arg.) Then talk to your husband and explain your fears. I don't think having a phone conversation with your husband and MIL will help. She's not FIT to care for your daughter if you feel scared leaving your daughter in her hands for even 8 hours.
I'm so sorry about your situation. You're a FANTASTIC mom. You're looking out for the best interests of your child. (And that's admirable.)
You'll figure it out. You know never to put your child in an environment, (especially for a week,) where she would be unsafe. This might cause the final rift between your MIL and you... but really? She's at the loss. Your husband will recognize that, and you can rest assured knowing your daughter is in good hands.
Good Luck?