I watched Evergreen's program as both a 747 guy and an aerial firefighter. Their problem, aside from not having an airworthy aircraft most of the time, was the expense. Tankers work on a daily availability rate, which is charged for each 14 hour workday of availability in any given 24 hour period, and also per flight hour. Evergreen's airplane was exceptionally expensive on both counts. Ultimately, the USFS didn't bite, and only Calfornia took it, and it ended up without a contract on a call when needed basis.
In the end, Evergreen represented it as available CWN, when it had no engines.
As far as the VLAT (Very Large Air Tanker), the phrase most often used is "different tools in the tool box." Presently the only VLAT is the DC-10.
The most common tanker in the national fleet is the Air Tractor 802 Single Engine Air Tanker (SEAT), an 800 gallon turbine airplane. More SEATs are operational and available than all the large air tankers combined. There's really no "strategic" vs. "tactical" wildland firefighting. Just firefighting. All the aircraft in the fleet work on fires that range from a single tree to tens of thousands of acres, desert grass to forest timber. It's not uncommon at all to have multiple aircraft types working the same fire, including the SEATs on the same drop line, fire flank, or target as the VLAT.