Whether you can prove it or not you can still file a police report. You have the vet to back you up and other evidence.
Yeah, about that ...
1)No one has seen the neighbor woman do anything
2)the vet, according to the OP, has no tests to prove anything
3)no one has proved it's intentional poisoning (lack of positive tests for other things doesn't prove someone is poisoning the dog)
4)maybe invisible voices in her head are telling her to do it. they can seem very real, I understand
5)if you tell the police she is throwing poison into your yard where your daughter is playing, they will react on it. they will ask her, she will deny it and then ask you how you know and your web of lies will get deeper.