Since OpenDispense2 hasn't run for a long time, there is a chance that an error will pop up that it cannot deal with (most likely on the coke machine). Here's what to do when that happens:
On Merlo:
/etc/init.d/vendserver stop /etc/init.d/dispsrv stop
Ensure that can ping cokemachine.ucc.gu.uwa.edu.au - if you can't, there's some sort of network problem that needs to be resolved, or the coke machine is turned off. You may need to open the first door of the coke machine and check there is a link light on the network port of the PLC (main board).
The snack machine is still running over serial - check which port it is connected to on Merlo, fire up minicom on that port, and check data is being sent both ways.
Finally, restart both services:
/etc/init.d/dispsrv start /etc/init.d/vendserver start