You can still do the fellowship: the h1b rules say you're not allowed to take other paid employment, but the fellowship will not be employment. Think of it as working for free on your project + winning a (very small) lottery :)
You can take vacation as much as you can take or go on a 8 week trip to India. I mean If YC takes you under it arms are you not willing to carve out 8 weeks for it?