Indeed, many of my fellow vendors are fun and smart people, some are also friendly :-). Sharing technology and sometimes even market insight with them - while having your competitive market positions and IP concerns in mind - is perfectly fine, in my estimation.
However, there are certain vendor behaviors that, in my world, can trigger a prolonged hatred. These are (ALL were observed in real world - some not by myself...)
- Super-unethical selling: fake references (I saw a vendor give a phone number of their own sales engineer as "here is a number for our reference government customer"...), direct and known lies, etc
- Hacking the competitor's website and other online resources (e.g. support site) or accessing them using stolen credentials
- Stealing evaluation gear/software from a side by side eval project at a prospect site
- Other examples of direct IP theft (once I saw this done using a VC firm as a cover)
- A few other things that SCIP folks would frown at.
UPDATE: fun follow-up post from Mike (and a bit more here) with more nefarious activities by vendors ... Yuck!