Pimples can be caused by a few things...
Hormonal changes - which may occur from not eating meat as it contains hormones
Stress - lifestyle changes, increased stress levels
Personal care products - even change in smapoo and conditioner can contribute
Diet - usually dairy, high glycemic foods and saturated fats
It is also connected to liver function. For more in depth information have a look at this link;
http://www.thefooddoctor.com/Acne-Ahealth_fdw_acne/
I am not sure why you have experienced an increased when you changed your diet, perhaps it is soy like someone else has stated? OR the change in hormone levels?
I am vegan and still get spots, particularly around 'that time of the month'. I have found the best thing for that is a blackmores herbal tablet called 'milk thistle'... it has nothing to do with actual milk, that is just the name of the plant. This plants aids detoxification of the liver. So from this I can see that my spots are as a result of poor liver function (I can thank my habits in my 20's for that one!).
The most important thing is to find the cause/s and work on that. Sometimes it can be a combination of things, remember the human body is all connected and our skin is our first line of defence against nasties!
Essential oils are brilliant as anti-bacterial agents and aid in the zapping of spots. ( I am studying aromatherapy and have my own skincare business)... Notably, tea tree lavender and lemon. Witch hazel toner is also great. This way you are using products from nature and not putting a myriad of harsh chemicals on the delicate skin of your face.
I hope my essay here is of some use! LOL
Best of luck with it, I know how painful it can be!