Are all Buddhists vegetarian? Most if not all Buddhists I know are vegetarian or vegan. Thinking about the precepts this is not surprising - one should abstain from killing. However is this universally true in all Buddhist traditions? Are there some Buddhist traditions that emphasize vegetarianism more than others? For instance I believe the Tibetans were originally pastoral farmers which wouldn't lend itself to vegetarianism naturally.

So is Buddhist vegetarianism universal, specific to tradition or culture, or a lot more flexible than that?
Very much not the case, so long as you don't engage in a No true Scotsman fallacy and say that they aren't really Buddhists unless they are also vegetarian. 

In some traditions it is common for a begging monk to eat whatever is put in their offering bowl, mixing it together first (see the interview with Achaan Chaa in Living Dharma).

For laypeople the provision is against engaging in the business of meat as it says in Vanijja Sutta, AN 5:177:


  "Monks, a lay follower should not engage in five types of business.
  Which five? Business in weapons, business in human beings, business in
  meat, business in intoxicants, and business in poison.
  
  "These are the five types of business that a lay follower should not
  engage in."


There's little that I am aware of, at least from the Pali Canon, that addresses meat eating for laypeople. Even monks get a pass in some circumstances on this as well, as it says in The Bhikkhus' Rules: A Guide for Laypeople:


  The Buddha did allow bhikkhus to eat meat and fish[88] except under
  the following circumstances:
  
  
    If a bhikkhu sees, hears or suspects that it has been killed for him,
    he may not eat it.[89] (M.I,369)
  


This is clarified with footnote 90 with the following cited quote:


  The flesh of animals which have been slaughtered to sell as meat for
  the people, however, is called 'flesh which exists already.' [It] has
  been slaughtered for their meat to be used for food by one person or
  by a group of people, apart from fellow Dhamma friends, or specially
  for the butcher himself… If people cook such meat and offer it to a
  bhikkhu, [it] will not be an offence to accept and eat it.


Citing The Entrance to the Vinaya, (Vinayamukha), volume 2, pages 131—133.

It should be emphasized that these are predominantly guidelines for monks, not for lay practitioners. 

This gets further modified depending on the specific tradition. His Holiness 17th Gyalwang Karmapa, simply try to lessen the amount of impact or the amount of meat eaten, and I have seen quite a few Vajrayana practitioners eat meat. 

Many other sects have de-emphasized the need for vegetarianism depending on the local conditions and the history of that specific sect. 

Basically: Even among monks the following of vegetarianism is not absolute and it isn't even clear that restrictions on that order exist for lay followers in a lot of different Buddhist traditions.