If you have a diesel running veg oil already, messing with steam is going to be pretty unproductive. Railroads moved to diesels because they were more efficient, AND you didn't have to keep stopping for water. Some of the last steam locomotives were turbines, and those could run condensers and solve the water supply problem, but the diesels still used less fuel.
That said, I loves me some steam power, and certainly "get" doing something just because you are of a mind to.
A roots blower probably is a poor candidate for a steam engine:
1) I can't think of a workable way to control the cutoff. To make an efficient steam engine, the inlet valve needs to close before the volume expands very much. Most piston engines can vary the cutoff using a Walscharts (sp?) or Stephenson valve linkage. This is the economical way to control the power output....A simple throttle valve just wastes most of the power in the steam. With just the rotor lobe passing the inlet, you can't vary the cutoff, and you will probably have to extend the housing to the point that the intake port is very small in order to get a low cutoff, and reasonable expansion and economy. That extension will need to fit as closely to the rotors as the current housing.
2) Seals on a roots blower are always a problem. They rely on close tolerances between the rotor lobes and the housing, and between the two rotors. When you add in the temperature swings a steam engine needs to support it is going to be a real problem getting it to seal well enough to be somewhat efficient, yet not crash.
3) The rotors are pretty massive and move directly from outlet (cool) to inlet (hot). This is a problem well known in reciprocating engines, and is partially solved by unilflow designs, but even those still have a problem with the piston "leaking" heat between exhaust and intake. The housings are aluminum to try to match the thermal expansion of the aluminum rotors...so the housing is an excellent heat leak between exhaust and intake as well.
