Its certainly a vexed issue for many practitioners and you'll find no shortage of threads on the subject here.
Firstly, plants are not considered sentient within Buddhist traditions.
To live and to eat, even as a vegetarian, is to subsist on the death of others. However, the crucial issue is whether the killing was done by you and whether that killing was intentional. If it is not intentional, then there is no kammic load.
What many people find is that after some sustained integration of the Dhamma into their daily lives and according to individual circumstances, there is a tendency to adopt a lifestyle which is closer to the ethical ideals embedded within the precepts. And for some people that may involve becoming less dependent on animal products.