I think this is a very difficult subject with grey areas.
I've been vegan for four years, and I have recently started shooting film--I continue to shoot film knowing that gelatine is one of its components.
I consider myself to be a strict vegan, however I have to be reasonable. If you attempt to divide certain actions and place them on a moral scale, I assure you that you will go mad.
I'll admit I've allowed film to enter my moral blind spot because I am comfortable with the choice and its implications. If there is a viable alternative to gelatine based film, I will discontinue usage of said film.
I try to reduce the amount of suffering to both non-human and human Earthlings, but I have to be reasonable to maintain a certain level of contentment in my life.
I don't necessarily condone killing--the murder of living organisms, whether sentient or not. I object to unnecessary slaughter and exploitation of said organisms.
If my existence is being threatened, by Staphylococcus or a wild animal to the extent I cannot avoid killing it, then I must kill it in order to ensure my survival.
As a vegan amongst omnivorous friends I've been thrown in to many wild scenarios such as being stuck on an island with a cow, fights with grizzly bears, the sudden change of heart in humanity when everybody decides to live a vegan lifestyle; and occasionally somebody feigns concerns for the wellbeing of potatoes and challenges my choice of lifestyle. All of which are unlikely to occur and miss the point.
At this point, whether plants are sentient within the parameters that humans are capable of assessing or not is irrelevant. I am unable to photosynthesise, so I must sustain life in a manner that is viable and most logical in relation to my circumstances. I reason that I'm doing less harm than I would be doing if I consumed products derived from animals. While I'm unsure of the exact number, it takes up quite a bit of grain and other resources to maintain livestock and produce a kilogram of meat than it does to produce and harvest various crops--for those with extensive knowledge on agriculture, please correct me if I'm wrong.