Have someone perform a risk assessment on the system - and focus on the quantitative aspects (ie what the cost to the community will be if it fails). Make sure that the contract has compensatory and insurance options in excess of those amounts, so that it is in the vendors 'hip pocket' best interests to ensure it does not fail. And of course make sure that the contract has provisions for review, should the potential impacts change or the vendor changes company name, is bought out, etc
You could also have someone do a thorough risk analysis of the system (google up the NIST SP800-30 document) as well as have them supply a complete inventory of hardware, software, and services they will be using to deliver the solution. Again, NIST have an online database where you can look up what vulnerabilities are known for some IT products.
Have the vendor perform a detailed risk analysis of the system - see what they think are problems, and what are not. Where you see gaps - ask them and see what color their faces turn.
Have a look around to see what failures or disasters you have seen in SCADA systems, refer those scenarios to the vendor, and ask them what technical measures they have taken to ensure that a similar act could not happen to them
You should also have your own people clarify and document their own roles and responsibilities with the system - don't assume that you have the resources on hand to manage your side of the situation responsibly - again a risk analysis will help out there.
And of course get it all in writing.