The best way to secure standoffs is to use plastic spacers and/or bearings. The force of the tightened screw will compress them and this will protect your screws from getting undone under vibration.
Here is an example of the standoffs that have white nylon spacers and metal washers on one end and bearing and piece of aluminum flats on the other. This works very well to keep screws secure and improves stiffness of the connection.
In very rare cases you may use Blue Loctite (available online, local hardware stores, or Walmart). Apply it in very small amounts and do not use Red Loctite, otherwise you will not be able to unscrew it without heat gun.