What budget did you have in mind? Outdoors is fairly harsh conditions, and most of the sensors for that environment are going to be around $60 each. And the sensor is going to be stolen along with the plant. So I’m not sure if that makes financial sense.
There are tilt sensors which are less expensive than vibration sensors, maybe $30 each.
What kind of plants are they? That is, specifically are they in a large pot? And if so, what is the pot itself made out of?
Also, any reason why you don’t want to just use a security camera in that area? In the long run that might actually be less expensive as well as showing you exactly who was tampering with the plants.
one option that might be feasible (depending on the type of put and area around the pot) could be to use a contact sensor with the magnetic piece attached to (or inside) the pot and the main sensor attached to something that is difficult to remove. this way, if it is stolen you are only losing a magnet rather than the entire device.