You can find the factory service manual specific to your Subaru vehicle including trouble shooting error codes and wiring diagrams over
This manual describes P0500 on page 2958 as related to either the ABS warning light OR a wiring problem, I've pasted the section below.
For part and loom ...
Typically, if the NSS failed, the motor would not turn over. Is that the situation here?
If you are able to start the car in gear with the clutch pedal depressed? There's also a clutch safety switch.
Black/Brown - motor oil
Blue with watery texture - Subaru Super Coolant
Red with oily texture - Automatic Transmission Fluid. The ATF lines hook up to the radiator on the front left of the hood. Also used for power steering
Subarus only like OEM thermostats.
Subarus only like OEM radiator caps.
It could be a clogged radiator or cooling system. Subarus typically get stop leak to prevent head gasket issues.
It could be the water pump. (Again, Subaru's like OEM water pumps)
The check engine light would typically stay off during overheating conditions. If your car is telling ...
There should be a programming button near the hood release lever. You should be able to use this to disable the alarm system and get your forester started.
P0500 on the H6 engine is, as you say, a Vehicle Speed Sensor. Check your ABS sensors on each of the front wheels.
Here's the usual cause for a P0500 error;
Check the ABS tone wheel on each front wheel, plus the connector wiring to it. The sensors themselves are usually pretty reliable unless they're damaged by removing the hub.