Baywood, do you think this was done as a random malicious act, or do you think that the plant has implications for your neighbour's garden? For instance, we didn't realise that fronds from our enormous climbing rose were leaning over and tapping on our neighbour's conservatory kitchen roof, and so they asked us to cut them, in fact offered to come round and do it, but my OH did it at once - all very amicable. A clematis montana is a prodigious grower, can weigh down a fence, or reach into a neighbouring garden. Does your neighbour hate that plant, or does she hate you? ie would she just as likely have poisoned your cat or slashed your tyres? Has she ever asked you to take the plant in hand? Does it shade her patio? Or attract insects she doesn't like?
I am not saying that her action is in any way justifiable, but if you do manage to save your plant, and I hope you do, and if you decide that escalating the conflict with legal/police involvement will only make things worse, maybe you could take avoiding action by making sure it is controlled on her side, or maybe asking her why she hates it so much, and if it is an issue of privacy, maybe think about other types of border marker, like a fence behind the plant.