Vibration monitoring is usually done on the motor frame at Drive End and Non Drive End in the X, Y and Z axes to detect for bearing problems and mechanical imbalances causing vibration - hence the 90 degrees apart. I would say that motor manufacturers don't normally provide as standard Vibration monitoring positions so when manual vibration monitoring is done - with a magnetic head sensor for example - the user is able to 'connect' in different positions and maybe the readings are not consistant. Some industries want constant real-time monitoring and you have some suppliers of this type of equipment above. On my last job I specified and got the motor manufacturer to include in the frames - Vibration Monitoring Pads. Basically, in the DE and NDE castings we had 3 threaded holes with permamnent screwed-in metal 'blocks'. This means that the manual vibration measurements are done on the same places everytime. If, in the future the customer wants to fit permanent sensors he will be able to. I'll check to see if I can find any norms or recommendations - but you could try your motor suppliers for their advice.