We recently received a call from one of our vendors. They wanted to let us know that one of our current employees had applied for a job opening at their company.
Was it unethical for the vendor to notify us?
Yes, it's always unethical to contact the current employer without permission. It's universally understood that you don't alert a candidate's current employer to the fact that their employee is job searching. The reasons are obvious and compelling. Virtually every manager with hiring experience will understand this and only a select few incompetent managers will breach this convention. Those few are also typically poor at hiring good people because doing this is a huge red flag and high performers have options and will usually self-select out after a stunt like this.
Now, if two companies have an existing relationship, then it's reasonable for the hiring company to alert the candidate's current company before extending an offer. In most situations this would be necessary as a courtesy and to "preserve the peace". Long-term contracts will typically have clauses covering this scenario. Consulting contracts always will. Some companies won't hire each other's employees, consulting companies will require a placement fee, some companies with close ties look favourably on this.
So basically, there's no way for the employee in this situation to proceed in the hiring process without his current employer being notified. But that's the thing: he could also withdraw. In this situation the hiring manager should have simply explained to the candidate that given the existing relationship, he would have to get approval of the move before proceeding with his application. At the point it's up to the candidate to give permission, and ideally to alert his employer himself, or to withdraw from consideration.
To summarise: it's unethical to alert the current employer as it jeopardises the candidate's job. It's unethical for the candidate or hiring company to keep the current company in the dark. At some point in the hiring process, the hiring company or candidate will have to disclose to the current employer that he applied for a job with a company that they have a business relationship with. When that disclosure happens will depend on a number of factors such as the level of the position and how likely the original company is to (dis)approve.
The point of the disclosure is not to alert the employer that an employee is looking to leave (which is an internal issue) but that it's bad form to secretly hire people away from a company you do business with. While doing so without "permission" is not illegal or doesn't breach a contract in most cases, it can seriously damage the business relationship.
Frankly, the guy should have minded his own business. I don't think the correct here is "unethical", but it was certainly in very poor taste.
People switch jobs due to various reasons. All of us have - at one point or another - been in the situation of smiling and nodding to our bosses while secretly feeling dissatisfied in some way, and looking for new employment.
Thus, for this guy to seemingly take it upon himself to punish someone for applying for a job is ... low.
I'm leaving my original answer up, because that is my opinion in most situations of this type. However, having read your linked question, my opinion has changed.
This Brent person sounds like a severely dissatisfied individual. You asked how you might make him see the light. The answer is you can't. He simply has something against you, personally. That's too bad, but it's time to let it go.
As far as the other employer calling your boss, his actions are now understandable. If he truly is friends with your boss then simply hiring one of your employees might come across as sniping people your boss has trained, and it could strain their relationship.
Had it been someone who was not personally acquainted with your boss the situation would have been different.
Was it unethical for the vendor to notify us?
A vendor-customer relationship is important. A vendor wouldn't want a customer to think they were poaching the customer's clients.
It's a tricky situation for a vendor to be put in the middle like this. I've worked on both sides of this situation and I can't say it surprises me at all that the vendor would choose their relationship with the customer over a job applicant.
It seems reasonable to me for a vendor to call the customer and say something like "I wanted to let you know that one of your employees has applied for a position with out company. Before we decide whether to interview or not, we wanted to let you know that we didn't solicit this individual, and to ask you if it would be okay to continue?"
It would be less okay if they simply said "Hey, your guy Jim Smith just applied for a job with us."
To be honest, I'm surprised that the company employee (Brent?) applied for a job with the vendor without first obtaining a promise of secrecy. Bad move on his/her part.
Many companies establish "non-compete" agreements with their suppliers/vendors in order to avoid "inside" information being used to recruit good employees after you get to know them. Most of these agreements permit individuals, acting on their own, to seek their own employment where they want, including at the vendor company, but prevent the companies from actively recruiting each others' employees.
It is very possible that the person who contacted you was trying to be completely transparent so your company won't have any reason to suspect a breach of the agreement. If this is the case, then the behavior is more along the lines "ethical to an extreme."