Congrats on your pregnancy!

Does your OB/hospital support VBAC? That's the first question, really. If they put lots of obstacles in your way and make it difficult for you then a doula may not help much. In any event, they can't stop you having who you want in the room with you. Generally speaking, having a doula does increase your likelihood of a successful VBAC, I think.

The best way to choose a doula is to ask around and interview some. You need to get a feel for them to know whether you're going to be able to connect with them - if not they won't be much use to you anyway.