ISO standards test for vibration resistance of electrical equipment
You can pay big money for any ISO specification so my first port of call would be MIL-STD-810F which you can download for free - at least you can use this to help develop what tests you want to perform. Here is also a wiki link to what it's about.

Here's an overview: -



I'll also add that I've seen this spec quoted for components such as crystals, mems and relays so it is relevant (and free as is the case with all MIL standards from what I can tell!)

There is also MIL-STD-202G worth reading. This document from Vishay lists several topics contained in that standard that may be useful.

Another approach is to target a few relay suppliers and see what specs they say their components adhere to and what tests they perform.