VMS - In the Bunker Rules, how do I set up a backup bunker?

There is no setup for it. When the vessel runs out of the bunker, VMS continues to consume the same bunker indicated in the bunker rule setup and reflect as negative ROB. The price for the negative ROB qty would be based on the Refill Price. If you want a backup fuel reflected on an estimate/voyage, there is a workflow to do this.

Refer to the following screenshot and estimate.

There is a negative (58.80) appearing at Port Rotterdam for HSBF. In order to use ‘Back Up’ fuel, you could input (-58.80) under the delta column for HSBF (triangle icon) and subsequent input 58.80 under LSGO. The bunker would then be adjusted as though the vessel was consuming backup fuel.