This is the exact same conclusion I came to from we-cannot-escape-the-heat-death argument. I will just make myself and my loved ones happy (latter because it indirectly makes me happy).
On an unrelated note, is your grandfather a preacher or your grandson?
I don't know that there's a way to escape heat death either, but my answer to that is: So what? I don't see why that should make me a nihilist. Nor do I really see why deciding to try to prevent suffering through veganism should follow from any of the above.
Instead I'd rather continue to muddle along my path. Leveraging what ability I can to make enough money that I can do whatever it is I feel like doing and end up going to bed happy. And that involves eating delicious things and having good times with the ones I love and doing drugs and drinking and contributing to charity and whatever else.
It doesn't, really, solve any of my problems. It doesn't, in and of itself, make me 'happy'. But I'm okay with it.
What makes me happy is the drugs.