"Dirty" oil is very normal for these engines, the oil will turn black almost immediately, it's nothing to worry about.

Regarding the failure to shut off, you are probably correct to suspect a vacuum leak. The manual shut-off lever is located on the engine (left side, on the "throttle" linkage). It is a metal lever with the word stop on it.

BTW, a runaway engine is not the same thing as a failure to shut off. A runaway engine is a malfunction that causes the engine to go to maximum rpm.