my neighbours have poisoned my Ivy and Virginia Creeper whilst I have been on holiday, I have reported incident to Police but have heard nothing.
I am sure this must constitute criminal damage.
I can see how upsetting it is to see a loved plant die, but so far you have told us nothing that proves it was your neighbour or even that it was poisoned deliberately. For a start you need to take the advice of 'The Prof' and get it tested to see why it died.
I am so sorry to hear what you are going through, we have the same going on with the people who moved next door to us. There are some evil people out there.