Think about ongoing support for Evergreen:

Unlike vended/licensed systems--where there were no options for support other than the proprietary vendor--and open source system can theoretically be developed by one vendor, installed by another, and supported by yet another.

Traditional alternatives for support have been "Turnkey" or "Software Only"--Turnkey supposedly meaning head-to-toe total coverage for the system.  In point of fact, "Turnkey" still means that you got only the support that the vendor was willing to provide and that you couldn't try to find another support source as an alternative.

KCLS suggests that you consider the the attached support matrix when you consider the various elements of supporting an OSLS.  Why not spend a little extra effort up front to more clearly define the roles and responsibilities of your vendor vis-a-vis your staff or other support sources.  Spell out the detail of who provides tier 1, tier 2, and tier 3 support for a variety of areas including database, hardware, application upgrades, bug-fixing, how-does-it-work questions, etc.

See the attached support matrix for ideas.  I will also attach a full copy of our RFP for future reference.